More general character-writing advice than game-writing advice (so not necessarily easy to apply to a mc):
Draw from yourself, or what annoys/fascinates you about people you know. Not the Mary-Sue-wouldn’t-this-be-cool approach but more like:
‘That was her fault, but she found a way to explain what happened that made it look like she was the responsible one’ What if I wrote a character who did this constantly, even fooling themselves into thinking they never made a mistake?
Grab something that doesn’t look like it fits, and see if you can make it fit. I think some of the most interesting characters are combinations of traits you wouldn’t expect to see together. I’ve got a fantasy villain who’s a horrible procrastinator. Doesn’t fit the looming dark lord persona, eh? Maybe a little silly? Maybe. But he’s had the potential to conquer the world for decades, and who dares disturb him enough to spur him to action? There’s not a Mcguffin holding him back, only himself.
Find the natural weak side of a strength. A careful, cautious tactician who can’t think fast enough in a fight. An excellent spy who has no friends because they won’t trust anyone. A brilliant scholar who melts if they fail.
Keeping asking ‘why’ or ‘how.’ Our scholar above can’t bear failing. How? Does she blame the test, or decide she’s insignificant and stupid? Or does she hide it by claiming it wasn’t important to her? Why? Pressure from others? (How does this show up in other places? Who has the most influence?) Internal high standards? (What is she trying to be? Why does she think it’s important?).
With different answers, we’ll end up with slightly different characters. One scholar claims she doesn’t care, trying to convince herself it wasn’t important. Another mopes when she fails, because everyone expects her to succeed and she doesn’t want to disappoint. (And does this connect to other parts of her life? Maybe she’s friendly and good at cheering people up, but never brave enough to honestly point out flaws, because she’s got to be a good friend as well as a good student.)
Contrast with other characters. Pick some you think might be too similar, and try coming up with situations where they’d have very different reactions (and then ask why and how!)
If you don’t know where to start, use a trope. Start asking why/how questions, or add something unexpected. I’ve had a lot of fun taking the Meyers-Briggs test as if I was one of my characters.
Well that was long post! This is one of my favorite parts of story-writing…