Background
Killua was born as the middle child to a family of renowned assassins, the Zaoldyeck 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 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. Meeting with Gon during the first obstacle, Killua saw Gon as a friendly competitor, and ended up becoming fast friends. The Zaoldyeck family are world-renowned assassins so rich and powerful that they are said to employ more than 200 people and live and own Kukuruu Mountain and the vast surrounding forests. They are so fearless that they have never hidden their whereabouts, and even tour buses take visitors to their "front gates" known as the Gates to the Underworld as those who enter never return.
Killua divulges that since his youth, he has been in constant training, having been left in the Celestial Tower (a tournament arena) at the age of 6. Swimming in pools of electric eels is amongst his everyday trials.
The grandmother, grandfather, father, mother, and five sons, all are assassins.
Personality
From the outside, Killua seems just to be the boy next door; cheeky, cheerful and full of mischievous ideas. 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, electrocution, and pain. Possessing both extraordinary agility and strength to match, Killua is one mean killing machine--definitely not one to be trifled with, as many of his adversaries had seen. However, until recently his greatest flaw was to fear characters more powerful than him, such as his elder brother Illumi. Killua had been trained as an assassin by his brother to be extremely cautious and engage in combat only if victory was virtually certain. This impeded him as a Hunter, as it is stated more than once that victory is never certain, and that depending on the circumstances, someone 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 prophesized that he would one day abandon Gon because of this. Biscuit gave Killua an ultimatum: Defeat Shoot for the right to go with Gon to fight the Chimera Ants, or leave his side. Killua lost, and resolved to leave after Gon had recovered from his fight, protecting him in the meantime. However, Killua finally had to face Biscuit's prophecy when he encountered a Chimera ant, while Gon (who couldn't use nen) wasn't far off and had no way of defending himself. Killua, preoccupied with his internal battle, was losing to the Chimera ant. At the last moment, Killua found that Illumi had planted a needle in his head with his ability, all the way back in the first Hunter Exam. This needle was responsible in forcing Killua to retreat whenever any situation looks potentially dangerous. Upon extracting the needle, Killua was freed from this restraint, and gained more confidence in battle.
Talents
Killua does not usually use any weapons in combat. Instead, he relies on his skills learnt as an assassin- hands that change into razor sharp claws at will, delivering a swift and fatal attack, that can rip out beating hearts or behead opponents in a split second; together with his ability to blend into the darkness it is a devastating weapon.
However, during Greed Island, he experimented with using twin yo-yo's, made of a special alloy, as weapons. The yo-yo's are extremely durable and heavy, and can be used in conjunction with his Nen ability. In the current arc, he is reluctant to reveal them to his opponents, as he wants the enemy to know as little about him as possible.
Nen Abilities
As a transformation type who can transform the nen into any substance at their will, Killua is busily practicing this ability. So far, he has managed to successfully transform his surrounding nen into an electrically charged form, producing volts of electricity. The inspiration for this has been attributed to the daily doses of shock therapy he received at home, a favorite torture device of his brother, Milluki.
Killua's nen attacks include the Narukami (Lightning Bolt), which is a bolt of lightning emitted from his hands, and the Izutsushi (Lightning Palms), where he electrocutes an opponent by touching them with his palms. He also has another ability known as "Kanmuru" (Godspeed). This ability uses the aura changed into electricity to make the electric signal/impulse travel directly to the body part he wants to react instead of going to the brain first, making the speed of reaction practically limitless.
Killua and Gon
Pressured by his fearsome big brother Illumi to say what he really wanted to be and to do - since he did not want to be either an Assassin or a Hunter - Killua averred he wanted to be Gon's friend, "hanging out" and "doing stuff" with him like a regular kid. Eventually, he got his wish. Killua and Gon became inseparable best friends, traveling companions and training partners. When they are not working to improve their abilities, they just spend quality time together.
