"Assassination—It's the family trade. We all take it up. My folks see me as an exceptional prospect. But I don't see that I should have to live up to their expectations."
Initially, Killua is introduced as a character who appears similar to Gon: cheeky, cheerful, and full of mischievous ideas. However, contrasting Gon's politeness, Killua can be quite rude to others, mostly strangers and older people. He is also one of the quickest-thinking characters and often analyzes a situation with great ease and accuracy. However, his ruthlessness and aptitude in killing show the other side of him—deadly, violent, and bloodthirsty. Conflicting with Killua's predisposal to kill is his unyielding loyalty to his friends and natural kindness. He claims Gon was his first friend, and he grows progressively more attached to him as time passes. 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.
1) He is nicknamed "Kil" or "Killu" by his family. However, neither the Zoldyck's employees nor Gon and his friends use this name.
2) Killua is the 3rd Transmuter in the Zoldyck family, after Zeno and Silva. All have silver hair as well, a trait found in all known Transmuters in the Zoldyck family.
3) Of the four main protagonists, Killua is the only one to have failed a Hunter Exam.