I’m going to assume you’re not talking about fictional characters (because I could name a few of those, with detailed essays to boot), which makes it a very difficult question. Outside of the world of fandom feelings, it takes a fair bit to make me angry, and the same applies to disliking people. Sure, I dislike those who abuse animals, who steal from elderly people, who don’t use indicators when they’re driving, etc, but someone specific? It’s hard to say.
If you’d asked me this a couple of years ago, I’d have told you that it was a girl I used to go to school with. We met in secondary school, knew each other for five years there, and we were fairly close. Then, at sixth form college, things changed (especially her). Long story short, she got it into her head that I was ‘pursuing’ her ex-boyfriend (boys are dumb, hello, and even now I spend very little time thinking about them), and then proceeded to make my time at college a bit of a nightmare. She’d bad mouth me while I was in the vicinity, making snide comments to herself and my friends, and trying to turn people against me, as lame as that sounds.
I was better than that and rose above it etc etc, but as a result, I found myself pulling back from my friends (which I guess had been her intention), and I basically stopped meeting up with them and avoided hanging out with them in case she was there. To this day, I speak to only two of the people I grew up with, and while it makes me sad at times, I can’t change what happened. I hated her for a long time, and blamed her for my loneliness at university, but continuing to hate her doesn’t achieve anything, so I just… don’t think about her anymore. Apart from right now.
I hope that answers your question! It’s a very boring response, I know, but all I have I’m afraid.