Now I'm not talking about their comic/tv/movie series, I'm just talking about the character themselves.
When it comes to a character you like or used to like, what's the one thing a writer could do to them or have become a part of their character that would almost instantly ruin that character for you and would make you never want to read or watch them again?
For example, for me it's when they give the main hero of the series a massive ego, such as the case with the Ben 10 series. Sure he had an ego when he was 10 but he became more modest and heroic during Alien Force, when he was 15. But the writers brought back the ego and while it was annoying in Ultimate Alien, it was insufferable during Omniverse, which is where I just dropped the series altogether. They just made Ben so damn egotistical and full of himself that it just made him completely unlikable for me.