The hair behind Tifa's right eye looks like it was shoved behind her eye socket. You could get away with this if Tifa were heavily stylized, almost to the point of abstraction, but since the rest of her anatomy is mostly grounded in reality, the hair behind the eye looks too unnatural.
An easy fix for this would be to make only the line work visible, so you can still see the whole eye but create the illusion of the hair being in front, making it look slightly transparent.
Another thing, just as a general tip (this applies to a lot of the other drawings as well), unless the entire iris is black, always give your character pupils. Without pupils the character looks lifeless or emotionless, no soul.
Also, her shoulders are a bit too broad... ok, I'll stop picking on Tifa now.
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The line work on the first one is weirdly pixelated. I don't know if that's a compression issue or the canvas is too small or the brush density is set too high, but the lines look very choppy. You've clearly got good brush strokes here so whatever's going on I don't think is your fault.
Check your brush presets and make sure they've got a slight feathered edge. It makes the lines look a bit more natural. Also check your canvas preset and make sure it's to at least 2400p (8.5 inch) x 3300p (11 inch) - portrait orientation. That's not to say anything lower is blasphemy, it's just a good resolution standard to work with.
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Last thing I wanted to say is your colors are fantastic. I can pick on the Tifa one for a while, but dang that glow is perfectly contrasted (I think more of it should be reflecting off of her body and hair, but I've picked on Tifa enough). My favorites out of all these are the first and fourth ones. Your coloring skills are clearly very refined and I love these because of it. You've got an incredible sense of scene temperature and color contrast.
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The only thing I'd say you should practice more is not your anatomy, but your proportions. In the six fanart section, Ramza's shoulders are HUGE, Cid's right arm and hand are slightly too small, Casca's right foot is too big (I know, it's supposed to be perspective, but it's too big in terms of where it's positioned in the scene), and Tifa's torso is too long.
You clearly know your anatomy, but knowing how to proportion that anatomy can make all the difference.
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Ok... I think I'm done now.