Killua Zoldyck is one of the four main protagonists and best friend of Gon Freecss in the series Hunter × Hunter. He is the middle son of the Zoldyck Family.Killua has spiky silver hair, very light skin and blue eyes. His eyes change depending on the mood that he's in, narrowing and sharpening when he goes into assassin mode. Killua's physique is excellent for a teenage boy , he is fairly lean at the start of the series, due to constant training and tortures he received when he was young. As time passes by and he undergoes many more hard trainings, he is much more muscular and toned, to the point that he developed amazing six-pack abs that are hard to achieve even by an adult. In the Nippon Animation anime adaptation, Killua's eye color is changed to green during the OVAs. He is also often seen holding a green skateboard (turned yellow in the 2011 series).
Killua typically wears baggy clothing, with his usual attire consisting of long-sleeved and dark-colored shirts. Most of his clothes have a turtleneck. Killua wears long, baggy shorts in the manga and Madhouse animated adaptation, but Nippon Animation's character design shortened them to reach above his knees. Killua also wears blue boots, recolored brown and black in the Nippon Animation adaptation.
Initially, Killua is introduced as a character who seems similar to Gon but different at the same time; cheeky, cheerful, and full of mischievous ideas. He is also one of the quickest-thinking characters in the series. He is confident in his own abilities, often being humble about it; but when somebody compliments him, especially Gon, he gets easily embarrassed. Killua is also known for having a sweet tooth, he loved everything sweet, as he spent almost 200 million Jenny on snacks when he was at the Heavens Arena. His favorite snack is chocolate, specifically Chocolate Balls.
However, his ruthlessness and aptitude in killing shows the other side of him — deadly, violent, and bloodthirsty. Having been tortured since birth, Killua has been conditioned to possess extreme tolerance for poison, electricity and pain. Killua possesses extraordinary agility and strength that makes him a one-man killing machine — definitely not one to be trifled with.
Conflicting with Killua's predisposal to kill is his unyielding loyalty to his friends and natural kindness. Killua often suffers immensely in his attempts to help his friends without a single complaint. He puts others before himself without being concerned about his own health or even his life.
Until recently, Killua's greatest flaw was his fear of those seemingly more powerful than him (one of whom being his elder brother, Illumi). Killua had been trained as an assassin by his brother to be extremely cautious that he only engages in combat provided that victory is absolutely certain. This impeded him as a Hunter, as Biscuit stated more than once that victory is never certain, and that depending on the circumstances, a person has the potential to defeat a stronger opponent.
Killua especially struggles with these dueling ideologies during the latter half of the series, as he begins to encounter stronger opponents. His brother's teachings remained firmly entrenched in his mind, to a point at which Biscuit prophesied that he would one day abandon Gon because of this. Biscuit gave Killua an ultimatum: Defeat Shoot to claim the right to go with Gon and fight the Chimera Ants or in case Killua lost, he resolved to leave after Gon had recovered from his fight, protecting him in the meantime.
Finally he had to face Biscuit's prophecy when he encountered a Chimera Ant while Gon (who could not use Nen at the time) was not far off and had no way to defend himself. Killua made an attempt to save Gon. While fighting with the Chimera Ant, Killua was also battling his inner self. Preoccupied, Killua was losing to the Chimera Ant. At the last moment, Killua realized that Illumi had implanted a needle in his brain way back in the first Hunter Exam. This needle was responsible for forcing Killua to retreat and jailing himself in his comfort zone whenever any situation would potentially endanger his life. Upon extracting the needle, Killua was freed from this restraint and gained full control of his subconscious to make his own decision whether or not to flee whenever there is a need to fight stronger opponents.