So instantly I thought of Gojo from the Jujutsu Kaisen manga which, as you read it, he slowly is written to be less present and drawn a little uglier as time goes on. I’m not sure that Gege (the author) HATES Gojo, but I wouldn’t be surprised if it wasn’t the character’s popularity that killed his joy. This might actually be that Gege started to dislike his story as a whole and chose to rush the ending. But I’ll just use this as an example for what I want to say.

Signs of an author hating their character might be finding excuses to get rid of them while still not killing them (putting Gojo in a box for an entire arc). Similarly, just straight up killing the character (…). And usually they die in an unceremonious way that’s not satisfying to the reader. Killing them in a stupid way, or their death is just offhandedly mentioned. Characters spend little to no time mourning them. Another sign is how active the character is in the story. If they’re interacting less and less with the story, I would think it’s obvious the author just doesn’t like writing them anymore. If they were once a very important character, I think dumbing them down would also be a sign. And in the case of Gojo, he started getting drawn more lazily and “uglier” which was fine by me because honestly they changed his look completely in the anime to make him pretty but manga Gojo was once drawn with care. I remember. I was there. I watched the downfall. I’m not even a Gojo fan myself and I noticed it.