Every character has to have flaws. Yes, even me (though I'll give you a clue: it's not my hair). We're all aware that I'm volatile, self-obsessed and don't play well with others, but while I prefer to see this simply as facets of my charming personality, Pepper delights in reminding me that they are, in fact, negative traits.
And when characters don't have flaws, we get the too-perfect character that every reader detests. They're known in some circles as Mary Sues, but if I start talking about those, Pepper will suspect me of having a new girlfriend or something, so we'd best stay away from that topic.
Note: a flaw is not the same as a weakness. A flaw is an aspect of a characters' personality that causes them to make mistakes or fail in certain situations, rather than a physical impediment that can cause trouble (like being unable to function without an arc reactor in their chest).
Characters have to have flaws, because it makes them seem human. Though, given the nature of this group, I'm guessing that's rather a misnomer. It makes them relatable. We care what happens to them (well, presumably you do: I'm not actually that worried) because we can see ourselves in them.
At the same time, you don't want a character who is nothing BUT flaws, because then nobody cares what happens to them. They're a failure and they're uninteresting. It's about balance.
Because I like talking about myself, and because I think I'm in a fairly good position to analyse myself, we're going to talk about my flaws. This is also partly because Pepper is sitting on my desk, complaining about them, and if I can't make her go away then the least I can do is turn them into a blog post. I should look into updating JARVIS to turn anything Pepper says into a blog automatically, come to that. Then I could ignore her and read it later.
Being self-obsessed. This can easily be seen as a defensive tactic and is usually found in characters who are used to having to look after themselves. However, it can also make them seem uncaring and they often offend other people without entirely meaning to, simply because they're too busy to think about their emotions. And when you've got Stark Enterprises to run, that's understandable. Unfortunately, that doesn't pass as an excuse. (Example of a fictional character who display this trait: Sherlock Holmes. Whom I greatly admire.)
Not playing well with others. You could also call this being introverted (though many introverts are able to work well in teams, and simply need time alone to recharge) or being unsociable. However, it's usually a sign of a deeper flaw: being stubborn. See, if your characters are like me, they'll be unhappy changing their plans because somebody else has asked them to. They won't take advice. This can be a problem when taking advice would save their life, and can result in mistakes. It can also link to being self-obsessed, when characters believe they're better than everyone else, and that's why they won't mix. It's a totally different matter when they genuinely are better than everyone else ... you know, like me.
I'm kidding. Kind of. But my hair is.
Fictional character displaying this trait: Harry Potter, who refuses to take advice on account of being the 'Chosen One' and believing that nobody else knows what he's going through. He overcomes it and takes Hermione's advice, though, which is pretty sensible. Hermione is like a fictional Pepper Potts -- organised and isn't going to let Harry's stubbornness get in the way.
Being volatile. Also known as being changeable or fickle. I would argue that I'm not that changeable, but Pepper would beg to differ. Characters who are changeable aren't usually very dependable; other characters will frequently feel betrayed because their feelings are not constant. They move on quickly, and loyalty isn't a large concern.
There's something to note when it comes to character flaws, however:
a tragic backstory is not the same as being flawed.
Characters who've been bomb-makers earlier in their life but who have repented and become a do-gooder can still be the 'perfect character' despite the angst that haunts them after what they've done, unless they have flaws to balance them.There's a difference between "ex-alcoholic who no longer touches a drop of drink" and "alcoholic who would like to quit but isn't mentally strong enough to make the effort", just as there's a difference between "ordinary person who betrayed their best friend and regrets it" and "ordinary person whose overwhelming sense of self-preservation leads them repeatedly to betray their friends and family".
Being angst-ridden is not the same as having genuine failings, which is just as well when you look at our team ... and Loki has enough angst that if it could be converted into electricity, we wouldn't need arc reactors anymore.
Note to self: investigate the conversion potential of emotion.
But while flaws are necessary, characters should recognise them in themselves and be able to 'grow' throughout the book to overcome their failings. I mean, I recognised a pretty major flaw -- being human -- and fixed that by building the suit. If that's not growth, I don't know what is.
-- Iron Man