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Killua vowing to protect not only his little sister(?) Alluka, but also Nanika, her wishgranting alterego. Especially since this was seconds after Killua told her to never come out, and reduced the poor thing to tears. After being called out on this by Alluka, Killua tells Nanika he was wrong and he'll always protect both of them, before tearfully asking for her forgiveness. She forgives him.
Kurapika revealing his powers to Gon, Leorio and Killua. The act of doing so was a great risk. One of the Troupe members, Pakunoda, had the power to read minds. If they were to be captured and Pakunoda were to read their minds, all of Kurapika's weaknesses would be exposed. Gon begs Kurapika to point a nen arrow at his heart so that it would kill him if he were to reveal his secret. Killua and Leorio follow suit, agreeing that it was a good option. Kurapika, however, refuses their offer. He says that he would not mind it even if they somehow managed to reveal his secret. He had good friends ..
Before this point, Kurapika is extremely depressed because he believes that the Phantom Troupe had been eradicated, eliminating his primary endeavor. Kurapika has not seen Leorio, Gon and Killua for six months, and soon he reunites with the others at a park. Gon and Killua are having an ice-cream eating contest when he gets there, and, on sight, it is obvious that Kurapika is upset. Gon cheerfully tells him that he should be happy. Now that he doesn't have to worry about the Troupe, he can comfortably recover all of the eyes of his murdered Clan. Then, as Kurapika gets over his shock, Killua promptly shoves a pie in his face. Enter Leorio, and Kurapika laughs for what might be the first time in a while.
At the end of the Greed Island arc Gon decides the first thing he's going to do when he meets his father for the first time? Introduce him to Killua: his best friend.
Chapter 339. That is all.
Meryem goes on a speech on how sociopathic he is and is about to kill Komugi, then he sees her being attacked by a hawk. He kills it and asks why she didn't ask for help, she answers she didn't want to bother anyone. It serves as heartwarming development of character for someone had previously being perceived as a complete psycho.
From the first chapter, all of the forest creatures giving Gon a farewell before he leaves for the Hunter Exam.
And then after that, there's Gon saying goodbye to Aunt Mito. Made even more heartwarming in the 1999 anime when he tells her that as far as he's concerned, she isn't just like a mother to him, she is his mother.
In the manga, when Gon is listening to the message Ging left for him and the recording says that if he keeps listening Ging will tell him about his biological mother, and Gon shuts off the tape without a second of hesitation, knowing that the moment he shuts the tape off it will automatically erase itself. When Killua asks him why, Gon tells him that he doesn't care about who his biological mother may be, because in all other respects his real mother is Mito (and unbeknownst to him, Mito is listening in and hears this).
In a weird, twisted way, the numerous measures Illumi takes to protect Killua. Despite his upbringing, he goes to very extreme lengths to protect his little brother.