Alessandro appears to be shorter, but skinny. He uses a sailor's hat, and wears purple clothes with white overalls.
Alessandro feels anger towards the Mario Bros., threatening them for how forgotten he was. And when he saw Luigi, he said he deserved to die.
However, when he was defeated, he asked for forgiveness. Luigi accepted, and Alessandro became friendly.
According to the Super Luigi 64 Manual, he's a loving person, always helping people towards some danger. Despite this, Alessandro becomes sad and irritated at the same time when forgotten, that's why he wanted to seek revenge towards Mario and Luigi.
Super Luigi 64
AfterTatanga tells Luigi to wake up, he'll be teleported to the 1995/07/29 Castle. You can only go to THE VOID, where you have to escape Waluigi. After you escaped him, text will appear saying "WHY DID YOU FORGET ME, LUIGI?" Then, Alessandro will show up, and the battle will begin.
He'll use a Fire Flower and will become Fire Alessandro and will throw fireballs at you. You have to dodge them, then hit him in the head 6 times. When you beat him, he'll lose his Fire form but will not give up.
For Phase 2, he'll use a Super Acorn to become Acornssandro and will fly at where you are. You have to Ground Pound him 7 times. Just like Phase 1, he'll lose his Super Acorn form but will not give up.
For Phase 3, He'll use a Mega Mushroom to become Giant Alessandro and will chase you. You have to use a Ice Flower to freeze, and then punch him for 10 times. After you defeat him, he finally give up. He'll lay down, and apologize to Luigi. He'll say he lost his mind and ask for forgiveness. You then have 2 options, Yes or No.
Saying Yes will result in Alessandro introducing himself before leaving. You can play as him when beating the game. Saying no, then Luigi throws Alessandro into lava, while Alessandro screams. Then, Waluigi will show up behind Luigi.