Van Silverlight said:
The most troublesome problem now is how to make him as humanly possible and with flaws and virtues? Granted, virtues can be done but flaws, how to present them to readers? Do I have to show that he is suck in specific areas that are non-combat stuff or do I have to show him that he sucks in specific combat stuff too?
I'm a little confused by your last sentence but I'll try and talk about this, and maybe I will help or maybe I'll miss the point.
I try to think of OCs by asking "what can they do that the normal characters can't?". Now, before you flinch at that, I think we all know the absolute WORST answer is "beat the bad guy cuz my guy is way stronger." I'm thinking more "what function does this character have that isn't filled by someone else"? And actually, setting this in Nanoha makes this fun because the actual main characters are, like, the strongest people in the universe so if your guy is too strong and too good at everything, it's more of a problem here that if this were some other series. But it seems like you're already thinking of that, which is good.
So basically, what role does your character play that compliments what everyone else does and doesn't just replace a position? Not really asking you to tell us, but just asking it for you to think about. If you like the answer, I think things will be okay there.
Now for the virtues vs flaws, don't feel like you have to have scenes that are just there to show how balanced your guy is. Let it develop and something can become a big deal if it needs to.
Also, for balancing flaws and virtues, think about this: I don't think anyone knows anybody who is the absolute best or worst at anything. Most people are mediocre at a lot of things with a few things they're good at and a few things they're bad at. Why does your guy suck at all these things? If your guy needs to be really BAD at something, you should think and justify that just as much as something he's really good at. And once you have a good character, the answers come quickly and everything makes sense.
Last, and this is a personal thing, don't write an OC bio and share it, don't spend paragraphs on how they look, just give a few details then let them start doing stuff, write what is needed (aka why they do what they do) and we can infer the rest. That is, personally, the best way to bring an OC in (for me). I hope this sounded good.