Killua has silver hair and dark blue eyes which shifted into green during the Greed Island Arc in the anime. Throughout the whole series, he is the only character who changes his outfit in every arc.
Killua was born as the middle child to a family of known assassins, the Zoldyck family. Showing great promise from birth, Killua has already mastered many killing techniques at a tender age, and is set to be one of the best assassins the family has ever produced. Bored and tired of killing, Killua unexpectedly rebelled against his family, injuring both his mother and brother, and ran away to join the Hunter exam just for fun. He met Gon during the first portion of the Hunter Exam and became great friends. He thinks being with Gon is exciting and adventurous.
Killua divulges that ever since his birth, he has been in constant training for the occupation of a professional assassin. Killua had spent years trying to perfect unique skills that assassins portray and possess. He is forced to battle in the Heavens Arena fighting tournament at the age of 6, which he took two years to get to the 190th floor.
Personality
From the outside, Killua seems just to be the rich boy; cheeky, cheerful and full of mischievous ideas. He is also one of the most quick-thinking characters in the series.
However, his ruthlessness and aptitude in killing shows the other side of him-- deadly, violent and bloodthirsty. Having been repeatedly tortured since birth, Killua has been conditioned to possess extreme tolerance for poison, electricity and pain. He possesses extraordinary agility and strength that makes Killua a one mean killing machine-- definitely not one to be trifled with, as many of his adversaries had witnessed and some are not a alive to tell about it.
However, until recently, his greatest flaw was to fear those who are more powerful than him, one of whom is 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 struggled with these conflicting ideas during the latter half of the series, as he started to encounter stronger opponents. However, his brother's teachings remained firmly entrenched in his mind, to the point that 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. Killua lost, and 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 couldn't use nen at the time) wasn't far off and had no way to defend himself. Killua made an attempt to save Gon. While fighting with the Chimera Ant, Killua is also battling his inner-self. Preoccupied, Killua is 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 jailed himself on 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.
In the first part of the series, Killua does not employ or use any weapons in combat. Instead, he relies on several assassination arts passed down to him through his family. These include adjusting his fingernails into claws and the Echo--a motion that creates an afterimage of the body through a varying cadence of steps. His eyes also change into a more sinister look.
The Technique of the Rotating Bodies: It is a deadly technique used in critical situations to create a succession of images on the retina, creating an illusion. It is a high level attack also called "Technique of the Dark Step" because of the total absence of noise during the movement.
Yo-Yos: During the Greed Island story arc, Killua begins using yo-yos made of a special metal alloy in combat. The yo-yos are extremely durable and heavy, weighing 50 kilograms each. They can be used in conjunction with Killua's Hatsu as well. Killua is reluctant to use the yo-yos in combat thus using only his raw physical abilities in order for his opponent to know as little about his combat abilities as possible.
Nen Abilities
As a Transmuter, Killua's most efficient in changing the form of his nen. As such, Killua developed several electricity based nen techniques. This feat was both inspired and made possible by the years of electric shocks he received as a child, both as training against torture and as punishment from his older brother Milluki.
Izutsushi (雷掌 Lightning Palm): The first and most basic ability Killua developed. As the name suggests, Killua is able to shock an opponent by touching them.
Narukami(落雷 Thunderbolt): A natural evolution of his Lightning Palm ability, Killua is also able to project a large bolt of lightning from his hands, striking an opponent with electricity from a distance. The name of the ability, Narukami, comes from the famous Kabuki play, Narukami Fudô Kitayama Zakura, in which the character Narukami transforms into a thunder god at the end of the fourth act.
Kanmuru (神速 Godspeed): Developed by Killua during his fight against the Oroso Siblings. With his hair, hands and feet changed into electric type aura, Killua's Kanmaru ability allows him to react and move his body at speeds that would normally be impossible (even in Hunter × Hunter universe). This ability changes Killua's aura into a field capable of transmitting all electrical impulses from his brain in a straight line directly to the part of the body he wants to use, completely bypassing his nervous system. It is made up of two parts: Denkō Sekka (電光石火 Speed of Lightning), the ability to move his body according to his own will, and Shippū Jinrai (疾風迅雷 Whirlwind), the ability to sense his opponents movements and have his body react automatically. He can also move from one place to another hastily, as he did when looking for Gon. Kanmuru can only be used within a specific amount of time. In order to use the ability again, Killua needs to be "recharged". He can do this by absorbing electricity through a socket.
Trivia
Killua is the 3rd Transmuter in the Zoldyck family, after Zeno and Silva. Coincidentally, all three have white or silver hair.
Killua's name in Japanese is written as "Kirua" in katakana. It combines the Japanese transliterations of the English words "kill" (kiru) and "killer" (kira).
Killua's eye color shifted: from dark blue it became green in the OVA series. In 2011 series, his eyes are back to dark blue.
He once stated that he doesn't know how to write letters, but he had written a "death notice, Zaoldyeck style" before. Another kind of letter is used for blackmailing people.
He hates red peppers. He was invited to Gon's house and Mito forced him to eat the red pepper but he spit it out.
Favourite Food: Kokorobokun (Chocolateball).
Killua occasionally is drawn with catlike features (paws, ears, mouth) in humorous situations, much like Botan of Yu Yu Hakusho (a previous work by the same manga-ka) did.
He has few nicknames given to him by his family members. His father and eldest brother calls him Killu, while his mother calls him Kil.
