Killua Zoldyck is Gon’s best friend and member of the infamous Zoldyck Family of assassins, having been trained since birth and conditioned to possess extreme tolerance for poison, electricity and pain. Although Killua fails during his first Hunter Exam by killing an opponent, due to his elder brother Illumi's influence, he passes his second after eliminating all other applicants in the very first trial. Killua remained by Gon's during the first half of the series until the end of the Chimera Ant arc. Killua's Nen affinity is Transmutation, which allows him to change the properties of his aura into electricity. Killua applies this to his nervous to developed his "Godspeed" Hatsu, which allows him to move at immeasurable speeds.