Character seems too flat?

Alright, well I'm writing a story (another one. Oh geez) and one of my MCs is extremely flat, 2-d, boorrrinnnggg! I've never had this problem before and all of my other characters in that story are well rounded and believable. Everytime I think of some traits for this character I just don't think they... show more Alright, well I'm writing a story (another one. Oh geez) and one of my MCs is extremely flat, 2-d, boorrrinnnggg! I've never had this problem before and all of my other characters in that story are well rounded and believable. Everytime I think of some traits for this character I just don't think they fit and I end up having to completely reform his character.

What I have so far is that:
His father died b/c of one of the major plotlines of the story
His mother is a drunk
His sister thinks he's insane b/c he believe in one of the being in the story (he's not the only one, there are a whole group of people)
He's quite but stubborn
Athletic
and... Thats it...He supports my other MC though. This is really bothering me. Is there any tips you guys have to help me form his character more? Thanks in advance
Update: Well, not necessarily is he 'boring' but he just doesn't seem believable.
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