Killua's Feelings
The nature of his feelings are often questioned, and it's understandable why people might see it either as pure friendship or romantic in nature. Either way, the fact he deeply cares for Gon doesn't change.
I'm on the side of seeing it potentially as romantic, due to the framing of many scenes.
This post will only include scenes from the manga, as I want to focus only on canon material as Togashi wrote it. I'll be using Japanese screenshots as the paneling and typesetting are important for my point.
We start to see the foundations of Killua taking care of Gon and watching over him during Heaven's Arena. The small ways in how he lies to Wing for Gon's sake, and waiting until he heals to continue learning Nen. Because he wants to experience things alongside Gon.
Afterwards, he wants to go to Gon's home to meet his Aunt Mito. He's starting to want to learn more about Gon. While on Whale Island, he's envying Gon for having so many goals while he himself is at a crossroad in life. There they decide to journey together until Gon finds Ging, and Killua finds what he wants to do, which he's quite happy about.
The first time we see Killua put Gon over himself is in Yorknew, when they're being held captive by Nobunaga. At this point in time, I don't think it's quite romantic, but it's the starting point for the readers to see how deeply he treasures Gon. Arguably, it can first be seen when he talks to Illumi at the end of the Hunter Exam, but to me that reads more as a pure desire to live as a normal kid and have friends.
Next we see during Greed Island how Gon has affected his life for the better. How he's the one who's glad he met Gon. That Gon changed the very trajectory of his life. What's important about this line is how the words frame them in the Japanese version.
"I'm the one who's glad that I met you."
The Viz translation did mess up here because in Japanese, he's mirroring Gon's exact words, when Gon said that he was glad to have met Killua.
And how the "Gon, I'm the one. Gon, I..." frames a panel of Gon only.
It almost feels like he's confessing in his heart.
One detail that's not in either anime is you can see he has an open-mouth smile in the manga. It conveys a lot more in how truly happy those words made him.
Then the moment he feels most needed by Gon, when Gon says that the only one who can hold the ball is Killua. You can see him smiling afterwards while telling Tsezguerra that he'll handle it. This is the beginning of what I think is a massive shift in his feelings. He's willing to happily give up his hands just to fulfill Gon's wish.
And then his shock when Gon says that the first thing he wants to do when he meets Ging is introduce him to Killua. He feels irreplaceable to Gon at this point.
Then we move onto the Chimera Ant arc, in which we get a strong focus on Killua's inner thoughts about Gon.
The first being after the Kite incident, when Killua notes how bright Gon's existence is, and if it's okay for him to stay by his side. His feelings are shifting again, now showing a strong desire to stay by his side.
What we start getting here are panels dedicated solely to Killua's thoughts. Blank, with only text, always framing the outside. The focus is only on Killua's feelings, and this feels very intentional.
After they lose to Knuckle and Shoot, Killua thinks to himself that he'll protect Gon, no matter what he has to give up in exchange. He's ready to sacrifice anything and everything for his sake. This is starting to exceed what's expected of a friendship, and Killua doesn't realize yet that this isn't normal.
"Gon, for the 30 days that you can't use Nen, I'll protect you. No matter what I have to do, and no matter what I have to give up in exchange. After that, it's goodbye."
The pattern with his thoughts in the paneling continues.
When they're talking about dates during the Palm incident, Killua shows no desire of wanting to experience a date. He only gets upset at the fact that he's going to have to separate from Gon, when all he wants is to forever stay by his side. At this point, it's starting to become clear that his feelings are transcending just friendship. Once again, we get a text-only panel.
After he says he's always been with Gon since leaving home, "And honestly, I want it to stay that way forever..." Then remembering the promise to Biske and how he failed to win against Shoot, "Oh no, I feel like I'm going to break. Even though I decided this myself."
And this panel, which could just be him wiping the sweat off from training, can also be looked at as hiding his expression from Gon. He doesn't want Gon to know what he promised to Biske, or that he was about to break down before the phone rang.
He always thinks deeply about Gon, but never lets Gon see it. And it's not due to something more typical like being tsundere (he really isn't one, he just gets embarrased a few times), but more likely that he doesn't know how to show that, with his lack of social experience. And also a bit of Japanese male pride in there, where they speak with actions more-so than words.
During Gon's date with Palm, he vows to protect Gon no matter what.
"Right now, Gon can't use Nen at all! I'll protect him during that time! Even if I have to sacrifice everything else in return."
Again, he's willing to throw anything and everything away for Gon. Gon is absolutely everything to him.
Then the scene where due to Gon's influence, he's able to overcome Illumi's control, and in the truest sense, break away from his family. It can be seen as just very profound friendship and a symbol of their deep bond. But it also shows that his life since leaving his family has been nothing but Gon. Gon shaped his life and gave it meaning.
"RUN AWAY" "He's my friend"
Many of his precious memories with Gon flash through his head, starting with their meeting.
Then this small detail, which is mostly a comedic moment, with Palm saying that because Killua was there, nothing went right (with Gon). This can possibly be seen as implication of both of their feelings, though they're unaware.
"That little snotty brat! He's been nothing but a nuisance! If only he wasn't here!! Then everything would have gone well!"
Killua is the only one who notices the icy abyss in Gon's eyes as they're just about to begin the battle against the royal guards. Nothing reflects in Killua's eyes except for Gon. He is his guiding light, even as he spirals into the abyss.
His hesitation to ask what Gon meant by "Lets go." when going to where Pitou was.
"If I ask, then I won't be able to stop it! There will be no turning back!"
He's afraid that if Gon doesn't give the answer he wants, that everything will break and no longer return to normal. He's so terrified of that that he's unable to ask. The thought of losing Gon is just too much to bear.
His deep sadness when Gon says it has nothing to do with him, when Killua is trying to calm Gon down in front of Pitou. His light is fading before him, but he bites those feelings back because keeping Gon calm at that moment is more important for Gon's goal. He swallows his own sadness for Gon. Everything is always for Gon, Gon, Gon.
Later on, Meleoron thinks about how sad Killua looked at that moment, when Killua tried to brush off that he just said he'd die alongside Gon against Pitou in the worst case scenario.
"Then why, at that time... Did you look so sad?"
Killua feels he is unable to live without Gon. There is no meaning to his life if Gon isn't there.
After all that happened, he believed he couldn't save Gon and broke down in front of Palm. But Palm assured him that despite his beliefs, what Gon needed the most was him. This was likely the only thread holding Killua together at that point.
Later on, when Ikalgo meets up with the group again, after Gon has left with Pitou, he realizes that staying behind means preparing for death. He thinks that Gon is cold for ignoring this and running off on his own, and starts that "We...", but immediately corrects himself to "No, Killua was prepared to die with you." Ikalgo had not been around for the previous events, but was able to read into how deep Killua's feelings for Gon ran.
Ikalgo sees Killua as a dear friend that gave him a new chance at life, but he's not thinking only of Killua or wishing to spend his life with him. Ikalgo wants to be of use to everyone, while Killua only wants to be of use to Gon. Killua is Ikalgo's benefactor, but his feelings are framed much differently. There's a clear line being drawn here between just being good friends and something more.
Afterwards, when Killua rushes to Gon when Palm notices something is wrong, his desperation to protect Gon is clearer than ever. When he yells out before Gon's final attack, in the Japanese there are exclamation marks after every word. This is something you don't see in the English version and can't tell from just the anime.
"You can't!! Don't do that!! If you use anymore of that power!! Just!! What!! Will happen to you!!"
There are stark parallels between Killua and Gon, and Meruem and Komugi throughout the arc. As Meruem gets closer to Komugi, he becomes more and more human. As Gon gets further from Killua, he goes down a deep, dark path, becoming less and less human. The former is seen as romantic--and it would be strange to parallel it if it was only meant to be a friendship.
Afterwards at the hospital, there's a lot of monologue from Killua about the events and how he feels about it all. Even after being hurt so deeply, his only concern is Gon, and getting Gon to realize what he put him through.
"I know... That Kite was your benefactor, and it has nothing to do with me. But... even so... I wanted you to say, 'let's beat them together.'"
And the last panel, highlighting the distance that has come between them.
"Was that as part of the team? Or as your friend? I know... that I'm just throwing a fit."
"I'll heal you no matter what!!" "But this time, I won't forgive you. Apologize to me? I'll save you no matter what, so..."
"I'LL MAKE YOU APOLOGIZE TO ME!!!"
A lot of the empty panels sans text, and framing text again. They really speak more about his feelings than his expressions.
He won't let anyone else but himself save Gon. After feeling so useless during the arc, his strong desire to do something only he could so for Gon was overwhelming. It was also the first time Killua ever demands an apology from Gon. After selflessly sacrificing himself for Gon constantly, he finally realizes that something needs to change, for the sake of both of them.
"The one who will save Gon is me!!"
After the events of the arc, it's only then that Killua remembers Alluka. He was solely focused on Gon to the point that even after removing Illumi's needle, he didn't remember Alluka until he needed her power. He rescued her from the Zoldyck mansion with the sole intent of saving Gon.
Hisoka says that Gon is Killua's most beloved when discussing the rules to Alluka's power with Illumi. It's not a word that has to be romantic in nature, but again the author is putting Killua's feelings only on display.
After Nanika saves Gon, he's still not ready to face him and goes off with Alluka. Even though he likely wanted to see Gon the most when he woke up, his heart was in tatters. It's then that he realizes Alluka and Nanika need him, and after struggling with the feeling that Gon didn't need him, it was the start of his journey to heal the deep wounds in his heart. He had to face the fact that Gon was more selfish than he thought, and would shut out anyone, even him, when things went south. Despite this though, Gon is more precious to Killua than he can put into words. But he's only 12 and doesn't yet know how to handle those conflicting feelings.
We don't see the apology scene, but it's clear that they chose to part with a smile, to wash away the sad feelings. The moment they both turn away, their smiles fade, with Killua noticeably looking more sad. He was leaving what made him the most happy for what he needed to do, which was protecting Alluka. He resolves himself to protect Alluka for life, both due to how dear she is to him, and because it was the only way to save Gon, even if it meant separating. He was able to find what he wanted to do thanks to meeting Gon. The impact that Gon left on Killua allowed for him to finally learn to love himself.
His small actions add up to show his love for Gon and how his feelings change over the course of the series. Gon went from someone who fulfilled his wish to live as a normal kid and have a friend, to an existence with a mutual need for one another, to someone that he'd risk anything to save, to someone who hurt him deeply. But his feelings for Gon were stronger than the pain, and there were far more happy memories than sad. His love is so deep that it wasn't shaken despite it all.
The framing of his scenes and thoughts feel intentional to bring focus to his changing feelings. Killua is the only character whose deep and intimate thoughts about another that we get to see. The framing is different from your average shounen way of showcasing deep, trusting friendship. The framing is not on them both, it's solely on Killua. It's showcasing Killua's feelings to the reader, and feels intentional to have the reader question if his feelings run deeper than friendship.
