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They can make or break your story. Especially if they're perfect in every way possible. (Smokin' hot, intelligently gifted, athletically inclined, etc...)

No one wants to read about someone who's a supermodel that is captain of every sport known to man. That's NOT realistic. Give your character some flaws because at the end of the day no one's perfect.

It's pretty boring reading about someone who has everything handed to them because... Well, they are superior to absolutely everyone they meet. The same goes for superhero fiction. Sure they can have powers but everyone has a weakness. (Superman's is Kryptonite)