fantasy - How do I round out a powerful character?

I've created a character named "Mathias Mindblade" who is trained by a wise wizard to lead a rebellion against the Darkness Empire and eventually to establish an Empire of Light.

Mathias has some severe obstacles to overcome and he will come into possession of some very cool powers. But I also want to be able to illustrate his humanity.

What I'm missing are ideas for his personality and character, and for some flaws or problems that might make him more interesting.

That last point is particularly important - in a previous question of mine, responses made it clear that readers have little interest in omnipotent, "Superman"-like characters. So what I want for Mathias is the kind of flaws that will keep him from being a boring Superman - but still allowing for him to have good points and a plethora of "bad-ass" abilities.

How do I choose what flaws to give him, and how do I prevent those flaws from overwhelming the likable, cool core of the character?

Answer

Suppose he's an invincible superman with no character flaws. But he's supposed to establish a kingdom, which means that after he waltzes over and personally defeats the Dark Empire army and kills its leaders, he needs people to support him--oh, and it's supposed to be an Empire of Light. Problem is, the power-hungry opportunists who were following the Dark Empire now want to follow him instead. And the idealists who were thinking about starting their own revolution paint him as a murdering dictator. Well...that's not too successful, now, is it? There can be plenty of challenges beyond simply defeating the bad guy. If you can't come up with an interesting plot even if he's personally perfect, then you should think more deeply about the consequences of his status and actions.

Now that you can foresee some of the problems that he'll have even if he's perfect, make him imperfect in ways that complicate things in interesting ways. He's supposed to found a kingdom, but he's afraid of or uninterested in public speaking; he likes to act, not talk. Or he deeply, deeply cares about his mission but gets arrogant when angry, and that tends to put off the very people who he's trying to rally. Or he underestimates his own power and thus is terrified of the power of the Dark Empire. (Or heck, maybe he's right to be terrified.)