Henry sees Rainer/Feather like something between a little brother first and a son later. Feather sees him, Jenny, and Micah as parents (although with Micah it's more in the sense of "you're my God mom's husband so by default, you're my other dad too), and he treats IV like a little brother. Feather is extremely happy he got to meet them because he's never really had family before (it's how he ended up here at 15, after getting in trouble for robberies to get himself by), so when he's shot and being ridden back to camp (on the back of Baylock), he even thanks Micah for letting him join. Somehow, despite him not really lasting that long, it was worth it to finally feel that kind of warmth for once.
Feather actually respects Micah a lot, although at the same time he wants nothing more than for Henry and Jenny to run away together and take him with them to be adopted. He knows Henry really likes her and tries to encourage them to be a thing.
He looks up to Micah in some way, but also knows he's not exactly the best partner either and that the situation here is messy..
Micah on the other hand doesn't care for Feather (especially because by the end of it he has that family bond with Henry and Jenny, and likes Henry more). Feather was always just a tool to him.. admired the fact he was able to pickpocket his gun so effortlessly when they met in Van Horn, but he just wanted him around for those thievery skills. He drops him off by the med den with a smirk knowing he's beyond saving and that it's going to hurt the two. Especially Henry.

Feather likes Mouse.. he thinks he's funny. Kinda has a similar thing going on like with Micah where he knows those two are kinda terrible but he doesn't exactly take them seriously either and sees something in them that he respects, despite Feather otherwise being a good kid you wouldn't really expect to tolerate those kinds of people.
Mouse finds Feather annoying though, but I think he'd become endeared towards him by the end of his life and feel upset that he's killed, too. Mouse p much starts off hating just about everyone except Micah and Warren, and grows to miraculously respect Jenny more by the end of it. Despite all the shit he's given her. I don't think he ever really cares for Henry, though. Henry's a bit too spicy for him to ever bond towards. Even if Mouse deserves it.

Little TL breakdown.. Rainer joins after impressing Micah in Van Horn by snatching his run and running off, Jenny snaps at Micah for letting a kid join, then proceeds to attempt to protect him by assigning him to camp chores rather than letting him go out on patrols. Eventually, Rainer becomes a medical assistant as Jenny's responsibilities and authority is growing a lot and she no longer needs the distraction of being Henry's assistant herself, tho the three work together in the medical den for a little while before she resigns that responsibility while Rainer is learning the ropes, or whenever they need an extra set of hands, she'll continue to assist if she's not busy with something else.
Micah eventually chooses Rainer to help him with a score, knowing Jenny wouldn't like this, so it's while both her and Henry weren't paying attention to where he was. It wasn't Micah's plan for him to get shot exactly, but alas. He's just not really upset about it either. And then the medical den and Hagen itself grows that much more quiet and empty. He brought a little spark of enthusiasm to a cold and dark place. It's the only scene where you ever see Henry cry. He attempted to save Rainer, and then goes out to bury him somewhere proper before anyone else could come along and snatch his body to burn. Comes back and breaks down into Jenny's arms.