In the Mega Man X series, Zero is Mega Man X's companion and a high-ranking member of the Maverick Hunters. Prior to this, he was a destructive, violent Repliod (Maverick) who would destroy any intruders who would dare enter Wily's lair where he was kept in. After wiping out Gamma's Maverick Hunter unit, Sigma himself went to face off against Zero. While the at-the-time commander of the Maverick Hunters took serious damage, he managed to knock out Zero after the gem on the latter's head began to glow with a 'W' symbol, disabling him and allowing Sigma to punch the crystal. During the fight, the two were infected with the Maverick Virus, which was contained in Zero's capsule, as they were in close proximity to said capsule when they fought. While the virus adapted to Sigma as he was too strong to be infected, the virus apparently made Zero good, as he was meant to be made as an evil robot in the first place.
In the first Mega Man X game, after X was badly damaged from Vile in his Ride Armor, Zero saves him at the last second. He is not seen again until near the end of the game; after X defeats all eight of the Mavericks, the two eventually meet up outside of Sigma's fortress. Zero goes on to face off against Vile, though he is captured (or left unconscious in Maverick Hunter X). After X is defeated by Vile, Zero manages to break free (or regain consciousness in Maverick Hunter X) and destroys Vile's Ride Armor with a point-blank ranged shot; this, however, was a fatal action, causing him to destroy his lower section. He ultimately dies in X's arms after the latter destroys Vile, though not before encouraging him to keep on fighting (and, if X did not collect the Arm Parts, gives him his Zero Buster).
Whatever remains of Zero was recovered by the Maverick Hunters, including his control chip, after the destruction of Sigma's fortress. However, Zero's head, body and legs were recovered by a group of Mavericks, the X-Hunters, who worship Sigma. They show up after Mega Man X destroys two of the eight Mavericks in Mega Man X2, who challenge him if he wants the parts of his deceased friend back. Depending on whether X collects the parts or not will lead to a different outcome that happens in the final stages; if all parts are retrieved, Sigma will show up with a black color-schemed Zero so it can fight X, but the reconstructed Zero appears and destroys it, allowing X to face off Sigma afterwards. If all parts are not retrieved, any parts X collected will be stolen before he heads off to the X-Hunter's base, and X will have to battle the reconstructed Zero, who is now a Maverick, in the final X-Hunter stage before fighting Sigma. During the battle, Zero ends up badly damaging the room he fights X in. After the battle, Zero will come to his senses, and will go on to destroy the main computer.
In Mega Man X3, the game marks Zero's playable debut, where he can be swapped between X in stages. He can perform charge shots, a slash with his Z-Saber, dash and wall jump. Zero can only be used once per stage, however, and if he reaches a boss door, he will automatically be switched back to X. Also, if Zero ends up dying, he cannot be used throughout the entire game. Story-wise, Dr. Doppler's Maverick followers attack the Hunter Base, with X being captured by the former Maverick Hunter, Mac. Zero then saves his best friend from the traitorous Maverick. After beating the Opening Stage, Zero call be called in at any time to swap out with X. In the second Doppler Stage if the player destroys Vile Mk-II, Zero can be called in the fight the sub-boss, Mosquitus. After the battle, Zero will be heavily damaged, and gives X his Z-Saber. Depending if Zero is still alive or not, he will appear in the ending, uploading the Anti-Sigma Virus-program Doppler created and striking Sigma with his Z-Saber (which has the program in it), causing Sigma to explode and seemingly destroy him for good.
In Mega Man X4, Zero is a fully playable character, controlling almost the same as X, though he uses the Z-Saber instead of a Buster. Also, instead of getting Special Weapons from defeated Mavericks, Zero learns new techniques, which do not cost energy (with the exception of the Rakuhouha). At the beginning of the game, Zero encountered his creator within his dreams, who ordered him to finally obey his orders and fulfill his destiny by destroying his nemesis. When he awakened from the nightmare (which is implied to have happened before), he was immediately dispatched to the Sky Lagoon colony which had been attacked by Mavericks. Repliforce was also confronted at the scene, claiming to assist the Hunters in evacuating the city; however, both factions did not trust each other, and, as a request for interrogation was declined, the Great Repliforce War broke out.
When the Repliforce were labeled as Mavericks after the Sky Lagoon incident, both X and Zero were assigned different missions to gather information about Repliforce's coup and to cut off their support lines. Iris assisted Zero on his missions as his operator, like during the Erasure incident from Mega Man Xtreme 2. However, Iris herself was part of Repliforce while her brother, Colonel, was Repliforce's second-in-command and therefore an imminent target for the Maverick Hunters. Iris didn't want the two of them to fight, torn between her care for Zero and her love for her own brother. When the two of them battled in the Ceremony Hall of Repliforce's command center, it was Iris who stopped the two Reploids from terminating each other by begging her own brother to stop since Zero had previously saved her life after the Sky Lagoon had crashed down. Colonel, who had always regarded Zero as a friendly rival, backed off. Zero, on the other hand, stated that someone had to stop Repliforce at all costs, much to Iris's grief.
When Repliforce's coup began, Zero rushed to the space port to stop the ascension of the military troops to the newly constructed space station of Repliforce, Final Weapon. There he battled Colonel, this time not holding anything back, just like his former friend, with Zero being the victor of the fight. After that, he traveled to the Final Weapon where he was, much to his surprise and horror, encountered by Iris, who was blinded by her grief of the loss of her brother and anger for Zero killing him. She had salvaged Colonel's Control Chip from his remains, fusing it with her own programming, thus resulting her body to transform into a massive Ride Armor-like combat form which Zero was forced to battle. Even though he did not mean to, Zero damaged the exterior power core of the structure, resulting in the destruction of the armor. He hurried to Iris's body, however, it was too late to save her. Whispering to Zero that she did all this because she wanted to live in a world for Reploids only, and that he could live in that world too. She died in his arms and Zero, who was totally overthrown by the pain, began questioning his own reasons for fighting and the means of his own existence. Filled with anger, he confronted General, leader of Repliforce, defeating him in battle. Upon the end of the fight, General realized that someone had taken control of Final Weapon, aiming it at Earth. Zero rushed to the Final Weapon control center, only to find Sigma waiting for him. Sigma reveals part of Zero's past at a time when Sigma was still the leader of the Maverick Hunters.
After Zero had destroyed Sigma and the General had sacrificed his life to make the Final Weapon self-destruct, Zero escaped the destruction in a small spacecraft, thinking about what he had done. He hadn't been able to save anyone he cared about. He had killed his own friend Colonel, the General, and Repliforce with him, who in the end has only been manipulated by Sigma and the Mavericks. Most of all, he lost Iris, who he had cared about for so long. Questioning whether or not Reploids were destined to become Mavericks after all, he returned to Earth.
In Mega Man X5, the space colony "Eurasia" was set to impact Earth, recently being coated in a large outbreak of the Maverick virus, which would cause catastrophes of unimaginable proportions. In the meantime, X and Zero faced Sigma on Earth and easily defeated him; however, Sigma lost on purpose to spread the Sigma Virus around the world. Due to this, thousands of Maverick Hunters turned Maverick, yet, the fall of the colony posed to be the bigger threat. In order to stop Eurasia's impact, X and Zero both would gather several enhancement parts to power up an old energy cannon, known as "Enigma", to destroy the colony before it could devastate the planet. While doing so, Zero noticed his strange reaction towards the Sigma Virus. While others (including X) got hurt by being surrounded by the virus, he himself actually felt energy growing inside himself; multiple infections of Sigma Virus anomalies even turned him invincible for a brief period of time. Behind the scenes, Lifesaver informed Hunter General Signas about his concerns regarding Zero, but also that there might have been a chance to develop antibodies from his DNA to cure the Maverick/Sigma Virus. Yet, the accomplishment of the Enigma was rendered priority one.
While the cannon only delayed the impact (story-wise; gameplay-wise, the cannon can destroy the colony), the space shuttle was readied to be launched and directed right into the colony to have it self-destruct right at the core. The shuttle would be operated by Zero, who volunteered because he felt that the world needed X more than himself. The operation succeeded, and Zero survived; however, debris would still rain down on Earth. Zero was later rescued by the Hunters, but the peace only lasted for a short time as a new threat arose from the debris of the Eurasia. A new virus, merged from the virus inside the colony and the Sigma Virus, called the "Zero Virus" by Alia, had begun to effect the impact area.
The area flooded by the new virus was strangely deformed and was filled with anomalies that even resembled Zero. Also, strange Mavericks were found in the area, bosses that actually resembled earlier bosses such as the Shadow Devil and Rangda Bangda. The closer X and Zero got to the source of the virus, the higher Zero's viral infection level rose, yet he still didn't show any Maverick behavior. Not wanting Zero to become a Maverick, X would fight his friend in order to have him brought back to Hunter Base. Eventually, the battle would end in a draw. Upon seeing X and Zero both completely exhausted after their battle, Sigma would reveal himself to destroy them. However, Zero managed to prevent this, protecting X and himself and forcing Sigma to retreat.
If the one who would pursue Sigma was Zero, Sigma would reveal to him that he let the colony fall on Earth solely to purify Zero's body with the virus to clear his mind and awaken his true self. Upon defeating his first form, Sigma would also reveal to Zero that he met an "Old Man" which knew a lot about Zero and his past, caring for him like a father and that it was that man's suggestion to flood Zero's body with the Virus to awaken him. After defeating his large battle body form, Sigma would then try to take X, who was still left unconscious at the scene of his battle with Zero, down with him. Zero would take most of the hit, seriously damaging him. The remains of Sigma attacked Zero and X one final time, impaling both of them through the chest area, but was then completely destroyed by Zero, who gained consciousness briefly and fired a charge shot at him from his buster.
Exhausted and damaged from the battle, Zero finally realized that he was going to die, and while in a state between life and death, he began recalling memories of his creator and his true purpose of existence. Yet, his memory circuits began to cease to function. After recalling his memories of Iris and apologizing to her, Zero asked X to live for him, bringing peace to the world. After X was saved and repaired by the spirit of Dr. Light, X searched for Zero, finding nothing but his Z-Saber, which he kept in order to honor his fallen friend.
As a non-canon scenario, if the Shuttle Operation was unsuccessful, the large amounts of the Colony Virus, together with the Sigma Virus Zero had already been infected with from Earth, would "awaken" Zero upon crashing on Earth, thus challenging X to a battle. His mind now cleared and caring about nothing but his mission to fight X, Maverick Zero battled X, resulting in a draw. After taking massive damage during the fight, Sigma revealed himself and prepared to destroy X. Zero, however, regained consciousness and he jumped into the line of fire, protecting X with his body, only to get mortally wounded in the process. When X destroyed Sigma in this scenario and was damaged badly, Dr. Light would appear to save him; however, Light erased X's memories of Zero and would implant a protection inside X's mind so no information about Zero would be accepted.
Mega Man Zero series
At some point, Zero realized that he carried the virus inside him and may even be continuing to spread it all over the world during his missions as a Hunter. Thinking that the world would not be safe as long as he was still active, Zero admitted himself into a research institute in order to get rid of the virus within him. At the same time, a project was started to analyze Zero as it had been revealed previously (during the Eurasia crisis to be exact), that Zero actually reacted positively to the Sigma Virus, and therefore might hold anti-bodies for the infection. Zero was sealed away and brought to a government lab. At some point, Zero's mind was extracted from the body, possibly for security measures in case something would go wrong while researching it.
Zero was later discovered in an abandoned laboratory by a human scientist named Ciel, who awakens him using a Cyber-elf. In turn, Zero saves Ciel from the forces of Neo Arcadia that was pursuing her and her comrades. Zero, however, was suffering memory loss due to "hibernation sickness". As it turned out, Neo Arcadia was the government of the world at this time period. Reploids were discriminated against and often deemed Maverick and terminated without justifiable cause. Copy X, a ruthless, conceited weaker clone of the real X, was in charge of this policy. Zero agreed to join Ciel's cause in fighting this oppression of Reploids. Zero would then go on to defeat Copy X's 4 Guardians as well as destroying Copy X himself. After the battle, the Neo Arcadian Core explodes, with the Resistance declaring that Zero was missing.
It is later revealed that Zero managed to survive the explosion, and is travelling through a desert while being hunted by Neo Arcadian Forces. He later succumbs to his injuries and collapses. When he wakes up, he is now in the new Resistance Base brought there by unknown means (later revealed to be Harpuia's doing), and meets the new members, including a new commander, Elpizo, who was obsessed with destroying Neo Arcadia. Elpizo executes his blitz-like "Operation Righteous Strike", despite the wishes of Ciel. During these events, Zero also learned what happened before his sealing; Zero himself helped X to end them by defeating and banishing Omega, the most terrifying "Reploid" of the time. Several decades after Zero was sealed, X sacrificed his body to stop the worst Cyber-elf of all, the Dark Elf, leaving him in a more ethereal form. X's body is destroyed and the Dark Elf is released by the now-rogue Elpizo. Zero defeats Elpizo, but the Dark Elf escapes. She and Zero appear to know each other, and X informs Zero that the Dark Elf was not always known by her current name. Her powers became a threat due to a curse by Dr. Weil, the man who started the Elf Wars, at which point people began calling her the Dark Elf.
Zero would eventually continue to help out Ciel and the Resistance in the last two games of the Mega Man Zero series, eventually facing off against Dr. Weil, the catalyst antagonist of the series. After the first of their two battles, the space station is damaged. Ciel pleads for Zero to return to Earth, lest he get stranded due to increasing transporter interference. Seeing Weil still alive, Zero passes the offer to go back and proceeds to finish him off in their second battle, ultimately sacrificing himself as the destruction of Weil's last form causes the station to explode. His sacrifice opens the eyes of many, finally bringing peace between the Reploids and humans left on Earth. The aftermath, however, leaves his friends grieving at his apparent death (his actual fate remains uncertain), especially Ciel, who is later inspired to pick up his work to create harmony between Reploids and humans.
Zero prominently sports a red color scheme. In his first appearance, Zero's armor was round for the most part with a red, black and white color scheme and white shoulder guards with a stylized "Z" insignia on his left shoulder. His helmet is horned and features a sharp blue crystal in contrast to his partner, X's smoother red gem. Perhaps his most striking feature is his long blond hair that flowed freely about, making Zero appear fiery in comparison to X's more cool and consistent blue scheme.
When Serges of the X-Hunters reconstructed Zero's parts in Mega Man X2, he upgraded the body, including reinforced shoulder plates and Z-Saber. Other enhancements were the improved arm guards (in contrast to the original streamlined ones) and leg joints, with improved mid-air dash thrusters and gold-plated covers, along with thicker and bigger chest armor. A small "backpack" device is also added as both storage and recharging unit for the Z-saber. The first version of Zero was apparently just an "incomplete" prototype, since this version is the outline shown in the blueprints in The Power Fighters.
Like X, Zero's power source is solar energy. His body material is comprised of a Titanium Z alloy, and is equipped with peculiar mechanisms called the Z-Brain (the head lens) and the Z-Heart (the chest lens), which conceal an unknown power.
Due to the artist character designer change in the Mega Man Zero series, Zero is given a design overhaul. His armor is lighter and more humanoid, and he's given a more streamlined helmet to present a much sleeker look. Unlike his previous form, Zero's arms cannot change into a buster form and instead keeps his weapons holstered around his body. The body Zero has in the Zero series is a brighter red than his original body exhibits.
Mega Man X series
Prior to his encounter with Sigma and becoming a Maverick Hunter, Zero was extremely violent and maniacal. He often unleashed feral screams and laughed psychotically, especially during his battle with Sigma.
Outwardly, Zero appears cold and emotionless to others, even towards partners X and Axl. However, it is shown that he has the capacity to care for others. Zero keeps his cool under most circumstances. No matter what his enemies do to him, he is always able to come up with a retort.
Despite not viewing himself as a hero, Zero has often displayed a tendency to sacrifice himself for the sake of others, seen in the first Mega Man X game, where he destroyed Vile's Ride Armor with his Z-Buster, which destroyed his lower body in half and ultimately killed him. He also shows distaste for innocent deaths. This is evidenced in his fighting against Neo Arcadia's ruthless policy against Reploids in the Mega Man Zero series; he even personally promised to Sage Harpuia that no more Resistance soldiers would come to harm as long as he was around.
Even though Zero does not openly show it, he too seems to long for peace between humans and Reploids. However, he has said that he doesn't have any problems fighting, unlike his more pacifistic friend, X; whether his enemy is a Maverick or a human, he will fight at his best. Another notable trait of his is that, despite his great power, Zero views X as the superior Maverick Hunter and considers him a hero, though he doesn't see himself as such. A prime example of this show of respect was in Zero's ending in Mega Man X6: when the scientist who was performing his sealing process asked what would happen if the world faced another crisis; Zero firmly said that the world is in good hands (X's), even when he is gone.
Mega Man Zero series
In the Mega Man Zero series, his personality remains mostly the same, though with some changes. Given that he's been in stasis, he is ignorant of some matters and confused about others, sometimes to the point he starts doubting or believing that he can't win. This is seen a couple of times. First when Ciel was kidnapped by a Golem and he was battling him with his Buster which was not doing too much damage, was slowly starting to lose hope until X gave him his Z-Saber back. Another example was when Dr. Weil told him he was a copy, causing him believe he was not the real Zero. He only gained his confidence back when X claimed he was indeed Zero, swapping roles so that X is the one that encourages Zero to fight instead of the other way around as it used to be during the Mega Man X series.
In Mega Man Zero manga, Zero is depicted as a Reploid having two personalities depending on his usage of a helmet; without his helmet he is portrayed as cowardly whereas the other one resembles his video games counterpart.
Zero is a very adept combatant on the battlefield. When he first encountered Sigma, Zero managed to heavily damage the commander of the Maverick Hunters. In Mega Man X, Zero was equipped with a powerful buster weapon known as the Zero Buster (shortened to the Z-Buster), but from X2 and onward, Zero wields a melee weapon known as the Z-Saber, a sword with a blade composed of pure destructive energy. This weapon displays Zero's great swordsmanship; he can perform lightning-fast slashes and combos with the Z-Saber, and while he cannot obtain performance-enhancing armors like X (making him a glass cannon), he can obtain special techniques like the Earth Gaizer and its many variations, Ryuenjin, Hyouretsuzan, and many others.
He appears as a playable character in this game. Mega Man and Proto Man discover him during a hostage case and Zero attacks them at first sight. When he realizes who they are, he joins them on their fight against other villains. He uses his Z-Saber as a weapon.
Zero appears as the final boss of Mega Man's campaign on Nightmare difficulty, and also the final boss of Bass's campaign in Mega Man 12. His design is a lot different than how it was in other games, due to him being unfinished during the time of his release. While both Mega Man and Bass were storming through Dr. Wily's stronghold, and also seeing them destroy most of his defenses, Dr. Wily had no other choice but to complete Zero and release him from his capsule, though he did not finish him in time, nor was his violent personality was sorted out. He was awakened from his capsule in a room Bass enters, and he immediately challenged him. Bass was easily defeated by Zero, though he is saved at the last second by Treble. Zero then goes into a room where Mega Man and Dr. Wily were, and starts to attack Mega Man while Wily retreats. Mega Man was also easily defeated by Zero, though he saved by Proto Man. Rush then takes Mega Man to safety while the Proto Man faces Zero. Proto Man ends up winning, though the battle severely drains his energy. Zero is then, presumably, put back in his capsule by Dr. Wily.
When encountered, the player should quickly realize that Zero does not have a health bar. As such, he is completely immune to all attacks dished out by the player, blocking them (even if the player manages to get a hit on Zero, it does no damage whatsoever, nor will it make him flinch). Zero attacks by perform a dash punch, punching the ground to release rocks if the player is above him, knocking said rocks into them, performing diving kicks, and firing shots from his Zero Buster. All of these attacks deal three points of damage. If the player reaches low enough health, the battle will end. The theme that plays during this fight is a remix of Zero's theme from Mega Man X2.
Zero appears as a Pair Unit teaming up once more with his partner Megaman X.
He is also a temporal Boss Unit when he is infected by the Sigma Virus.
Mega Man X series
“X, you shouldn't expect to defeat [Vile], he was designed to be a war machine. Remember, you have not reached full power yet. If you use all the abilities you were designed with, you should become stronger... You may even become as strong as I am. I'll scout ahead and collect as much information on Sigma's fortress as I can. I'll meet up with you when you get there. See you later! X, I know you can do it!”
— Zero to X after he saves him from Vile, Mega Man X
“Let's split up. I'll go in first, and then you can slip in while I keep the main defense forces busy!”
— Zero to X when they meet up at Sigma's fortress, Mega Man X
“Stay back X, I'll take him on!”
— Zero before facing off with Vile, Mega Man X
“Don't listen to him X! Go ahead and blast him!!”
— Zero telling X to shoot Vile, Mega Man X
“Maybe.....but I'm not through yet!”
— Zero before he destroys Vile's Ride Armor, Mega Man X
“X, I've taken too much damage..... Auto repair systems can't handle it... My power is fading fast... Your power is greater than I thought. Maybe you can destroy Sigma.”
— Zero's dying words, Mega Man X
“You are more powerful than you were before, but Sigma is much more than he appears to be. You're going to need an edge. Take my arm cannon, and your attack power should increase. Good luck, X!”
— Zero's dying words before he hands his buster to X, Mega Man X
“Sigma, you should have studied the blueprints closer! There is only one Zero!”
— Zero after Sigma tells him he was destined to follow him, Mega Man X2
“Sorry to keep you waiting, Mega Man X. The greetings will have to wait. We've got to defeat all the Mavericks, including Sigma! I'll take out the main computer, and you follow Sigma. Let's go!”
— Zero to X, Mega Man X2
“Yea... I guess so... Sorry to cause you so much trouble.”
— Zero to X after the latter asks him if he remembers him, Mega Man X2
“X, you need to go after Sigma! I'll try and destroy the main computer!”
— Zero to X, Mega Man X2
“I'll be OK. Get going, X!”
— Zero to X, Mega Man X2
Mega Man Zero series
“I never cared about justice, and I don't recall ever calling myself a hero... I have always only fought for the people I believe in. I won't hesitate... If an enemy appears in front of me, I will destroy it!”
Since the revealing of Zero's origins in Mega Man X4, a popular fan theory was that Zero was possibly involved actively in the classic Mega Man series timeline after its events, from either spurring the creation of X, being activated in his earlier, violent and destructive personality against Mega Man, to being the cause of a "cataclysm" that marked the end of the classic series. However, Keiji Inafune officially stated that Zero did not activate during the classic series and had no active role on the events that transitioned to the Mega Man X series whatsoever.