Oni, later known as the Fierce Deity, is a minor Termanian deity that possesses unspeakable power.
As revealed in Majora's Mask 2: The Untold Legend of the Fierce Deity.
Though Oni and Majora were created to be protector deities, meant to serve the people, their protective instincts led them to defy their masters. They, despite instructions by the Four Giants to do otherwise, bothered and harassed the mortals; Oni himself appeared to them as an angry god and demanded offerings and worship. The early Termanian people first gave him the name "Fierce Deity."
Majora and Oni competed so ferociously for power and influence that the land and the people itself were in danger.
Thinking quickly, the Four Giants stripped the duo of their powers and imprisoned them within the bowels of the earth, a system of caves called Four Giants' Catacomb.
Alas, even that did not stop them. Though initially powerless, Majora and Oni escaped their prison and traveled through Termina recollecting the powers that they had lost.
At full power at last, Oni resolved to destroy Majora once and for all, and, though they battled fiercely, none could best the other. The battle wrought terrible destruction and suffering upon Termina, and, just as the people cried out for help in their last hour, the Four Giants once more appeared and this time sealed their creations completely within inaminate objects - the Fierce Deity Mask and Majora's Mask.
Both masks, though hidden carefully, were eventually discovered by mortals, and used for their benefit. The Fierce Deity Mask fell into the possession of a young hero thousands of years after the battle, and, though he used it in battle, the Fierce Deity was unable to take full control of his body through sheer force of will.
After the Hero of Time saved Termina from imminent doom, the mask was lost to history, and what became of the Fierce Deity is unknown.
As he was created for a single purpose, Oni has a rather one-track mind. His complete and utter disregard for the lives and feelings of anyone other than himself makes him a liability on the Four Giants' part.
The only thing Oni understands and respects is power and battle, and is arrogant enough to refer to himself as the most powerful being in the universe, insisting that Majora is in every way inferior, and undeserving of life.
Despite this, Oni has a great respect for the Four Giants, though he cannot understand why they care for such "weak" creatures.
Though usually quiet and calculating, Oni is savage and brutal when need be, and has a short temper when dealing with those he considers inferior.
He often does not cease attacking a target until it has been rendered completely and utterly silent and unmoving, and he isn't above using tricks in battle.
Oni is about eight feet tall, and garbed in a white and blue, though he does wear an armored breastplate decorated with various symbols, as well as armored gauntlets. He has the distinctive Hylian-style pointed ears, usually attributed to his skill in magic.
Oni's eyes are a milky, pupil-less white, and he sports face paint in a blue 'v' shape on his forehead and red face paint rimming his eyes and reaching down to his cheeks.
Oni's hair is silverish-gray, and worn in a messy style. He wears brown boots that reach just below the knees.
Oni is a deity, and as such, his affinity for magic far exceeds any mortal's.
Oni's greatest weapon is his large, two-handed double-helix sword. Though it is extensively powerful, Oni cannot carry a shield with it. The sword can fire rapid bolts of light at far range.
Oni can perform multiple, extremely acrobatic feats; flips, extremely long bounds, and when properly powered up, can hover and cross air for a limited amount of space. Oni has the timestop ability, though its name is slightly misleading; instead of stopping time completely, he can slow it down for several seconds.
Oni also has the ability to generate a magical shield that allows him to absorb up to three hits with no damage done.
Oni can create multiple, and perfect, copies of himself, though each of them contains only a fraction of his power.
Oni's eyes have the power to show him illusions, and he can sense the presence of other magical beings.
As a last resort, Oni can charge and release his essence in bursts of energy; these blasts are so powerful that anything caught within their range is instantly incinerated, though it severely weakens Oni, and if used before Oni's system can properly recharge, he can end up on the verge of death.