KIllua, what can I say about him? He started off as a happy-go-lucky person, kinda like Gon, but as the episodes went on, the story tells of how messed up Killua's life and family are. His ability to carry on even though his whole life was made up of so much pain makes him strong.
People like him because he's super badass and OP (Which he is), but he became my favorite character because of his character development. As the episodes went on, we slowly see Killua change from being the kill-everyone-that-pisses-me-off, to a gentler, kinder soul.
Also, his loyalty to Gon is amazing. At first, Killua didn't have friends. He just killed when told, yet he changed and became a person that would protect his friends instead of just protecting himself.
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Even when Gon hurt him during the Chimera Ant Arc, by saying that Killua doesn't understand the pain of losing someone, Killua still chooses to stay by Gon. Instead of hating Gon and having a big fight over it, like many people do, Killua chose to support Gon in his own way.
Overall, I think that Killua has amazing character development. He went through so much, yet can still stay by Gon's side. I love him for that.