I used to hate Dawn from Pokemon for this reason.

I don't, and honestly I find it stupid that people do.
I knew people that hated Shiemi just for "getting in the way" of the very non-existant Rin x Yukio ship because Rin likes her. In fact, I met a Rin roleplayer who hates her guts just for that, when she's the sweetest and most innocent character on that whole damn show without being bland or extremely annoying.
Ugh.
Another thing is hating a character simply for being shipped or close with a character you like.
Like, I fucking love Shura even though she's shipped with Mephisto a lot and Mephisto even seems to take some interest towards her. But I don't care. That shouldn't stop me from liking her character.

And while I shipped Nu and Ragna together so much, I actually don't mind Rachel shipping with Ragna (I would rather shipped with Rachel and Ragna together instead of Noel/Celica and Ragna). Even though I was a bit triggered when Ragna hugged her and Rachel blushes.

Yes. I hate Hiyori for this reason.

It's one of, if not the single most stupid reason to hate a character.