- The character has no flaws. (I've always found Beth in Little Women a little too good and prefer Jo with her more obvious faults!.
- The character doesn't seem real (having no flaws contribute to that!).
- The character's virtues are not believable. Usually because they're exaggerated or, conversely, the character has been portrayed as all evil with no virtues at all. I can only think of one character where total evil "works" and that's Sauron from The Lord of the Rings but there not much is portrayed about him. You see what led to him losing the ring but not what caused his overriding lust for power. Given the focus of the story is on Frodo Baggins, this portrayal of Sauron is fine (we are seeing the Dark Lord from the hobbit's perspective for one thing). Had the story focussed directly on Sauron more would have been needed to show his character. (I doubt even if Sauron was born totally evil).
I see character failings as not the opposite of their virtues as such but the reasons why a character doesn't work. Some of the reasons are:-
I'm Allison Symes and write fairytales with bite, especially novels and short stories.