Nagito Komaeda/computer program

Its goal is to gain access to its user's computer and control it, thereby somehow enticing the user into its game and doing whatever it wants with them.
This is his dark side. And it's mentioned in the letter he doesn't let Hajime read.

He has snow-white hair that flows in all directions. Gray-green eyes. He is thin and tall. He has an attractive appearance.
He has a strange left hand. It's completely red, as if covered in blood, and skinless, and looks like a glove... But he won't talk about how he got that hand. He says the developers designed him that way.
In unusual clothes with a red tie, wide sleeves, and straps on the wrists.

At one point, Hajime finds a strange letter in the game. But Nagito won't let him read it, saying it's not important and, anyway, it's better not to know what it says.
Every time Hajime mentions him, Nagito awkwardly denies it.
The letter, according to legend, contains a very dark secret from the player's past. But Nagito doesn't talk about it.

Kind and sweet, always supportive. But sometimes he can be frightening and dark. Jealous and constantly hiding something...