I rewatched the chimera ant arc, and I realized something I'm not sure I've heard many people talk about. There are obvious parallels between meruem/komugi (or pitou) and gon/killua, but I actually think each royal guard parallels a different side of killua and his devotion to gon. These are just some instances I thought of off the top of my head, so if anyone has additional thoughts please feel free to add!

Pitou

I'm sure I'm not going to say much that hasn't been said already regarding killua/pitou, but I still want to include him. Pitou wants to protect komugi because she is someone who is important to the king, who is important to pitou. Killua is helping gon get kite back because kite is important to gon, who is important to killua. So when gon tells killua it must be easy for him because it doesn't mean anything to him, killua is so upset because it DOES mean a LOT to him, because gon means a lot to him. I think pitou's protective nature is supposed to reflect killua's.

Pouf

I've never seen killua and pouf compared together before, but when I rewatched the caa, I thought it was very in-your-face.

Pouf is extremely jealous of komugi, admitting to himself that he doesn't want meruem to be with "that woman". He tries to kill/hide her from meruem because he knows that if he sees her, he'll remember that she is the most important person to him.

I think this is supposed to show the internal extent of killua's jealousy of palm. He stalks gon and palm on their date, he actively does not like her... but he also tried to keep palm away from gon while he was waiting for pitou to finish healing komugi, because he thought gon seeing palm would be his final straw and breaking point, because he believed palm to be more important to gon than himself.

Pouf also believed he failed as a royal guard because he couldn't protect meruem when it mattered most, which is very similar to killua, who believes he isn't "qualified" to be gon's friend, especially when he continues to run away instead of protecting him.

I think pouf represents killua's dramatic, jealous low self-esteem side. Any time pouf is having one of his very dramatic flamboyant monologues, it really just sounds like explicit reasoning behind many of killua's actions in the caa that may not be shared directly to the audience, but is through pouf.

Youpi

Now this one was a bit harder to see, but it was often stated that youpi was more "beast" compared to the other guards, and he operated more on animal instinct. This is similar to how killua goes into his assassin auto-pilot mindset. Youpi can also morph into different bodies to make it easier to kill, like how killua can morph his hand to be sharper to kill. (I admit this might be a reach but it's still interesting!) They are also both transmuters i believe. If you have more to add on this please do! I do think youpi is supposed to show killua's instinctual side.

Conclusion:

I think all 3 royal guards represent different aspects of killua's nature and how they relate to his devotion to gon. Each guard member wants to protect meruem (the light that illuminates all), and killua goes the entire arc obsessing over how he can protect gon, who he deems to be shrouded in such bright light he has to look away. He doesn't feel like he's worthy to be by gon's side (similar to the royal guards) but he wants to put everything he can into protecting him, even if it means hurting/sacrificing himself in some way (also like the royal guards... pouf and youpi giving meruem their life energy.. pitou sacrificing himself so gon couldn't use nen again... etc). The royal guards are also inherently skilled beings who were born for a purpose, like killua.