Dialogue
Okay, I think that's difficult. Most writers make, at least, decent characters. I think an unbearable character is not necessarily a bad character, just unbearable, which might be the intention of the writer. Is it really bad?
I'd say a bad character would be something that goes against the grain of its own self. This is so difficult; not even Gary Stus or Mary Sues fit the mold of a bad character. Because most of the time, stories with overpowered abilities or just “perfect“ characters, usually have struggles that try to undermine some of their abilities, I guess. I'd say, something that goes against the grain of the character. So, let's just say the character's happy, and at the same time sad, try to combine this in a way that doesn't make any sense. It just feels like the character is paused or something like that. I don't have any examples.
I don't deem any character as bad, but perhaps, not what I would add to my character.