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The guy on the right seems to have an odd/incomplete head shape. The eye should be at the vertical center of the head. If I move it to there, here's how I figure his head shape should be...

His head is raised and tilted, though... if his head is on an angle that would have his eye appear higher/farther away, then I think you'd need to have the bottom of his chin showing or something.
Anyhow, if this is his head shape, his hair is too tight around the top-left. If you're going with your original head shape, I think his hair isn't entirely covering where his head would be.

For the girl, I think her left arm (our right) was too far to the right, so I scooted it in a bit. I also moved the skull on her shirt over to the left more to match her angle better.
Oh, and maybe her jaw line on our right should have less of a slope. It should go like, under her ear, I think. But not connected to her ear - there should be space where the jaw goes up or whatever. I don't really know anything about skull structure, though, so I can't really explain that too well/accurately.

Anyhow, that's what I think needs changing/improvement. I like how you did the guy's mouth and nose. Whenever I try to draw more realistic noses/mouths, they always look weird/ugly, but you did it well.

Hmm... maybe the way the hair spikes up makes me think it shouldn't seem too close/flat on his head. I don't know exactly how big the forehead is when it comes to face/head proportions, but I think he maybe doesn't have enough forehead, or his head would seem too short if it's wear I'd assume it would be (assuming the hair isn't that flat on his head). If I draw an outline of his head shape as it is, it still seems to bump out at the top-left... But yeah, it's not like everyone has the exact same proportions. It personally looks odd to me, but that's not saying much.

I think the girl's collar bone/neck should be centered with her chest, though. I assume the skull is on the center of her top... I guess I sorta made that change in my redline without entirely noticing. By moving her arm in, I centered her head or whatever. I think she'd have to have a very thick neck for her neck to be considered centered as it is, but that thing around her neck suggest that's not the case, and even then the skull wouldn't be in line with her angle.

You don't have to do anything you're uncomfortable with, of course, and I could have some things way off since I dunno what the rough draft looked like. Some further opinions could be of use... I just try to post in every topic that has 0 replies so the artist can get some kind of feedback, even if I'm not 100% sure about what could use some changes.