A lot of my reasons for disliking or hating characters is pretty petty so um...
Bored and Umbilical Cord: Not Barst.
Vyland: Boring while also having my least favourite design among his other three companions.
Johan: Punchable face, kind of annoying, and I don't really care for movement in FE4. Goodbye loser.
Hilda: Very sure she is supposed to be hated, and boy do I hate her. Doesn't help that I really like Tailtiu and her kids, so knowing what she did to her truly does anger me.
Sylvia: Please stop interfering me from pairing Lewyn with Erin thanks.
Lester: I really don't like his design. I'm not even entirely sure why, I just find it awful. I'd gladly take Dimna over him any day if only I could also get to keep Lana.
Alec: Not a fan of his design, nor his personality, nor his smug face, and I never found his combat exceptional enough to like him more.
Cain: The Leonster trio (Cain, Alva and Robert O.Speedwagon) don't really have much of a personality in the game, and iirc they don't even have any dialogue (exception being the death and escape quotes). So yeah they're boring, but at least Alva and Robert look cute. Cain not so much, sorry dude.
Oujay: That blue and yellow colour scheme is awful to look at. He can do fine in combat at least, but Dick and Edgy are just way better, so that doesn't help him.
Raigh: His portrait in-game looks so bad, and he's pretty average in terms of gameplay for me at least. I also hate it whenever I get him in FEH.
Cath: Haha yes I do find it very fun to talk to you three times in three separate chapters just to recruit a thief with poor base stats considering the join time and the fact that two other, way better thieves join fairly early on.
Kent: I don't really remember much of his personality, but his combat is usually awful for me, especially when compared to Sain and Lowen
Wil: He just seems so...generic. His backstory is interesting at least.
Heath: Wtf is that hair. Disgusting.
Matthew: I used to like Matthew at first, but I've slowly started to just dislike him and I'm not really sure why. I suppose two reasons is that I don't like his design as much anymore, and Legault is just better in every way that I don't see much point in using him.
Colm: I don't really dislike any of the SS cast. Colm I like the least I guess, mainly for his usual attitude towards Neimi and his meh design. Still, I don't completely despise him.
Selena: I have played neither Fates or Awakening, but judging from her dialogue in FEH she seems like a tsundere type. Not a fan of that trope, so I assume I wouldn't like Selena much in the mainline games she appears in either.
That's all for now