wowee!! I love the third one! Miku!!!!

amazing ! but there is no signature on some drawing

Ok. Here goes. xD
Mostly Specific:
The heads are shaped weird, especially the sideview-kissing one. Guy's head looks squished.
The pikachu-girl's hand doesn't look like it has fingers and the red-vampire-girl's folded am looks like it ends up like a spiked weapon.
The warrior girl's sword-hand is placed weird, if someone actually held a sword like that, it'd fall off. (tip: grip a stick. Copy your hand. In other words, have the fingers curl and the thumb curl over the rest from the other side.) And the hand-on-hips (I think) is looking more like hand-on-stomach.
The wedding dress girl's dress is drawn on weird. You might be trying to make her twist sideways? But it looks more like she put on her dress sideways. Or backwards because of the corset-thing.
General:
They tilt sideways to the left. Did you notice? And the clothes aren't well thought-through.
Their feet are misshapen. Some of them, at least. And the clothes are sorta strange looking.
Work on perspective, some of them look wrong because of how some parts seem bigger when they're supposed to be smaller (the last girl's hair.)

Your not bad either. The flattened-on-the-floor-tounge-out-girl looks awesome, so I think the others might be older ones? Generally, your a good artist. Clean up your digital work and don't go overboard with the shading<3

15 hours O.O
Worth it though :)