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.
For me it was Henry. At first i thought he was annoying and i couldn't stand him. Then i started using him a lot and got a lot of his support convos. Now i can't help but to not put the guy on my main team.
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No one. I either liked them from the start, disliked them from the start, or liked them at first but came to dislike them later.
This. Though, to be fair, that's probably because if I dislike a character, I'm liable to immediately bench them and never get to know them. Meanwhile, I get plenty of chances to grow to dislike the characters that I start off liking.
Ruler of the Unknown Universe "Courage is the magic that turns dreams into reality." - Emil, ToS:DotNW