That's funny to think about! Well, let's see...
Nate's is his negative mindset. I'd like to say it was his impulsiveness, but it gave him a boyfriend lmao. With all his negative thoughts he'd aways think no one likes him and he deserve to suffer.
Matt's is the way he never shows how he trully feels, he'll aways hide it. He starts to show it to Nate, with him Matt starts to feel secure enough to cry when he needs to. But to his best friend, Mike, he mostly just tell what is happening because they have this kind of brotherly relationship, but he never goes too deep in the sentimental stuff. He hides so much that he started to have night terrors since he was young.
Mike is... well, if Matt hides stuff, Mike is like a pandora box. He keeps so much to himself that he began to have anger issues and it's pretty easy to annoy him and make him punch you. Even to Matt he never say a thing.
Charlotte is too naive. She believes in everyone and never says no, putting herself in bad situations. Even after a traumatic experience, she kept beliving in people.
I think that's it? I think about those being their "defects", things they have to work through.