For me, the most important thing about a character is that they interest me. I don't necessarily have to like them. I don't mind disliking the as long as they interest me. The worst kind of character is one who I feel absolutely indifferent towards.

That's not to say that they have to have an interesting personality: characters with dull personalities can interest me, if something interesting happens to them or they're involved in an interesting situation.

People have talked a bit about the importance of originality in characters. For me, characters don't have to be original. I'm not sure I've ever encountered a truly original character. Everything's based on something. Originality is neither here nor there: what's important is that a character interests me.