One of our favorite ways to start building a character is to choose a flaw we haven't explored yet. Without flaws, it's difficult to create conflict in your story that feels authentic and organic, and there's almost nothing worse than reading something that feels completely fake. We all know we're here for fiction, but we want to be immersed while the story unfolds, right?
Flaws are what make characters compelling, far moreso than their virtues, in our opinion. It's like when you look at that perfect person who never seems to do any wrong, whom everyone is half in love with - you're just waiting to find out what their damage is, right? Similarly, characters with no flaws or with cop-out flaws are difficult to relate to, and experiencing the story through them can be an extremely frustrating experience.
When you choose a flaw, pick a real one. They don't have to be obvious in the sense that their flaw would make them a terrible person, but it has to be something that will affect the character's interactions with others and their environment in a negative way. You can take stereotypical virtues and make them flaws very easily, actually, if you just take them too far!
Take a character who is nurturing, for example. The instinctive move might be to say that she cares too much (gag) and it only hurts her in the end, which could very well be true and be interesting if it's written well, but this is often just a cop-out flaw. If you want to go with a character who really does care too much, make them be overbearing and demanding in it. Make them run roughshod over their loved ones' decisions for their own good; make them sneaky and underhanded when it comes to making sure everything turns out for the best, because they honestly know best, don't they? They had good intentions. They just wanted to see their loved ones happy, so they took matters into their own hands - and then make it blow up in their face, and make them learn some hard lessons about autonomy, consent, and respect.
Flaws don't have to be things that make your characters act evil. You can have honesty be a flaw when it's brutal and uncompromising - when your character won't keep confidences and so can't be trusted, because their ethical code means that they will tell the truth even when it hurts, because honesty is the best policy. Generosity is a great virtue, but a character who is generous to the point of giving the shirt off their back and then comes to resent the people they give and give and give for - when they never asked for anything in return, and always insist on giving for the high that they get from it - is a flawed character. And that's awesome!
Our best advice for coming up with believable flaws, if you can't or don't want to start with a flaw, is to look at your character's virtues. Think about flaws that would go hand-in-hand with them (generosity/resentment, nurturing/controlling, etc), or ways that the virtue could be taken too far (honesty into brutal honesty, or untrustworthiness) and that's a great place to start!