From their recruitment scenes, I thought all the kids were intolerably insane but most of them grew on me. This does however have the effect of making Severa seem a lot worse as she viciously rags on the other kid's insane quirks. Now that I find those quirks endearing instead of insane, Severa just seems meaner in comparison.
I also hated Virion by virtue of him being an archer but I started getting overwhelmed by Pegasus Knights, Wyvern Knights, all sorts of things with wings and suddenly I can't get enough of Virion.
Virion. His limited dialogue through the campaign makes him out to be pretty obnoxious, but I actually liked his supports a lot. I also hated Jafar initially, but his whole plot with Nino changed my mind pretty quick. --- On a different website I'd say "I'm surprised you got modded for insulting Hitler." but not on GameFAQs. - CatfishWiggins
For me it was Jill. I can't stand racism, in any form, and her first line of dialogue made me grit my teeth in anger. But then as I saw her base convos and her supports I saw that she was changing, and she eventually became one of my favorite FE characters.
This, actually --- PSN: Fredfredburger51 GameFAQs, where everyone trolls everyone
From this game, Lissa and Maribelle. Brady and Gerome a little bit. Still don't like Severa, but maybe she grows on me when I see a few more of her supports.
From other games, definitely FE7 Wallace. Was annoying in Lyn's arc, but his supports later in the game were surprisingly great. Also FE8 Neimi I guess, although that had more to do with her being RNG-blessed in my playthroughs and less to do with her personality growing on me. When a character trucks everything that gets in its way, you tend to overlook the character flaws a little more.