The most important thing is to add traits that are detrimental to your character. And things that are annoying are not necessarily enough to keep your character from being perfect. Take Bella Swan from Twilight, for example. She is an awful character because there is literally nothing she does that is actually a flaw. Sure, she's clumsy, but is clumsiness really a negative trait? It can be annoying and it certainly isn't helpful, but it isn't really a negative. Of course, when it comes to Bella and the whole series, there is a significant negative in that they're insufferably boring and poorly written, but that's on the author, not the character, so not helpful.

Basically, you need to make sure that the character sometimes gets in his/her own way. He or she needs to drive away allies or get themselves into trouble. Maybe they don't think before speaking, or they refuse to change their mind about something even in the face of evidence. Maybe they care more about being right than the feelings of other people. Or maybe it's an overabundance of a good thing. Maybe they want to help people so much, they become cloying and overly intense. Maybe they're so giving, they don't know when to say no and end up getting taken advantage of. Maybe they're a complete door mat.

A lot of people tend to think not making a character perfect is giving them less than likable traits; that isn't necessarily true. It's good to be kind and giving and want to help people, but those same qualities in overabundance become extremely negative and detrimental to the character. So, yeah, a character must have traits that are truly detrimental in order to be well-rounded and interesting. Otherwise they're just Mary Sues.
-Quaddy

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