Accelerator is the most powerful Psychic in Academy City. His real name is unknown, though Accelerator recalls his surname was composed of two characters and his given name consisted of three characters. First appearing as an antagonist of the third light novel, he later becomes a main character and the lead of the science side in the Toaru Majutsu no Index series.
A sadistic boy about the same age as Tōma, he has an ability called "Vector Change", a form of telekinesis allowing him to change the vector values of anything he touches due in part of having a thin coat of AIM fields surrounding his body. This ability allows Accelerator perform feats such as reversing the blood flow inside a person's body, increasing the wind velocity around him to create a massive plasma storm, and blocking sound. His ability is automatic, which means that he must consciously allow gravity to affect him and that his body blocks all ultraviolet radiation, resulting his albino appearance (white hair and pale skin) because his body is unable to make melanin. The automatic nature of Redirection also prevents the use of projectiles (like bullets) and explosives (like bombs) against him.

Killua is a member of the Zoldyck family, who are a family of legendary assassins. Since birth, he trained endlessly in the art of assassination. He eventually ran away from his family and profession and joined the Hunter exam. He befriended Gon almost immediately and has loyally followed his best friend since then.
Killua is usually a kind-hearted and playful boy but once provoked he becomes extremely violent and bloodthirsty. This seems to be a trait he got from his father, who believes that trait would eventually make Killua return home.
He is also from the transformation nen group. He constantly strives to improve his nen ability, developing many interesting techniques as time moves on.

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Accelerator