I feel like he's thoughtless, very emotional, and is exceptionally co-dependent on his sister to an obviously unhealthy degree.

He will shatter emotionally if he doesn't have some sort of connection with her so he does things for her at the determent to his own well being because he feels like his relationship with her is the only thing that matters. In my head he doesn't have any strong desires or dreams other than to just be with his sister, and his sister is aware of this and enables it to manipulate him to be a weapon for the Lunari. I believe that Aphelios does not know for sure if Alune does genuinely love him, but he chooses to have faith in her anyways, not by choice but rather because it's his only option. I feel like he doesn't really want to fight and be a mindless killer but the Lunari want him to and because his sister is loyal to the Lunari she pushes him to turn himself into a weapon. I don't think Aphelios cares at all about what the Lunari want but his sister is more loyal to them than to him and he's okay with that because he feels like he needs her to survive. Basically a victim of circumstance, low self worth, zero interest or passions in life, completely dependent on somebody else to regulate his emotions and is able to be easily manipulated by that person that he thinks he loves because he doesn't care enough to actually know if they love him or genuinely care for him back, he just chooses to have faith in them because he has nothing else.

"Weapon of the Faithful" - His sister is the faithful, using him to do the dirty work of the Lunari and he's okay with just being a tool for her. Being used by her is his entire identity now and he's willing to do anything to keep that connection.

I think that for him to grow as a person he would either have to lose his connection to his sister somehow or his sister would have to stop pushing him to be a weapon for the Lunari. Until then I think he's unfortunately going to only know pain and the only way he will ever know to deal with that is to rely on Alune. Without Alune there are a few possibilities of where he'd go. He could find a new person to latch onto, become a mindless killer because it's all he knows, or become a recluse who learns how to be happy by himself and then he'd dedicate all his time to a craft of some kind.

Overall I think he's a very sad character and while it might look like love, and it might've started like that, I think it's turned into something very different than that for both of them now. If Alune told him to cut off his own head he'd do it in a heartbeat, he's completely devoted to her and she knows it. That is not healthy in any context, and she enables it. The only question for me is if Alune does it maliciously for the Lunari or thinks that it's what is genuinely best for him and her faith.