Lightflame
Ashfur is my least favourite character, although Hyperadam's reasons already perfectly explain my hatred for him, so I don't have much to add there.
I also dislike Brokenstar, because I thought he was an awful villain, and Silverstream, because she was pretty much a living plot device, and a jerk to boot.
Owl Feather as well, for being really annoying in the short time she showed up. I also don't like Yellowfang's father, Brackenfoot, because he felt like a “sports dad”, who only loves their kids if they're bringing pride to the family.