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 out of boredom and joined the Hunter exam to pass the time. He befriended Gon almost immediately and has loyally followed his best friend since then.
Killua is usually a kind-hearted and cheerful, cheeky boy, but if provoked, he becomes extremely deadly and bloodthirsty, able to kill an enemy in seconds.
He is confident and aware of his abilities and strengths, and not afraid to speak his mind, but also smart enough to know when to hold his tongue. He can get embarrassed easily when someone compliments him or openly shows affection towards him.
He is very intelligent and quick-thinking, and skilled in analysis, particularly induction. He is able to calmly and rapidly form strategies to adapt to stressful situations.
Due to his training as an assassin, it’s an ingrained habit of his to be extremely cautious when approaching a fight, often assuming the worst case scenario, so he is quicker to flee than to try to face an apparently stronger opponent. For most of the series, this is his greatest flaw. He is also a loyal friend who is selfless to a fault, and suffers greatly trying to help his friends without a single thought to his own health.
He is also from the transmutation Nen group. He constantly strives to improve his Nen ability, developing many interesting techniques as time moves on.
He is also the third person to have a transmutation Nen as his father and grandfather also are transmutation Nen users. Coincidentally, all three family members have silver hair.