These are great! I do have some minor nitpicks, though.
-Frisk's hair seems a bit too round on the front view.
-Flowey's petals are a bit lumpy looking. The anti-aliasing between the petals and his face doesn't work either.
-Overall, your colors are a bit too muted and close together.
I did an edit to demonstrate the last point with Sans.
Yours -> Mine
Most of your palette seemed to be from the same hue, which killed the contrast imo. If the washed out look is what you're going for, then disregard this.
(though in that case, you may want to lighten the outlines to match)
Oh, and a super minor nitpick- Sans only does the one-glowing-eye thing during his boss fight, so if you want him to look friendly I'd add the second eye.