Looks great.

Would love to see some sparkle effects around ice king. Ice king's expression feels wrong for his character. Like he's contemplating. Ice King always seems to be manically bipolar. Excited then depressed, but rarely is he slightly vexed.

Marceline isn't too clear as that specific character right now. Maybe it's the hat preventing a good read of her hair part in the middle, or her sharing a skin tone with other characters. Maybe her hair should be full black or visible as longer (over a shoulder or something instead of hidden behind the back so it looks shoulder length). She seems to carry her hair long and heavy which pulls it straight down, it isn't light and wavy as I see here.. Looking at reference Marceline always shows 2 fangs, you are showing 1.

Fire Princess is reading as a small boy. I think it's the fact you've made her face the most angular of all the characters when her face in the cartoon is quite round. Her hair is also less feminine in your version. She also seems to have a short neck in your version. In the cartoon her neck is thin, maybe her head could be raised slightly? Not sure.

I also might not have immediately read Princess Bubblegum if I hadn't seen the other more obvious characters first. You're using I think one of the more rare hairstyles for PB. This is the style that's on young PB, but your character looks facially more like older PB. I'd suggest swapping for either the long hair or the pony tail which are more iconic mebbie?

Really good stuff.

Alright, a few quick adjustments. I gave PB what I feel is a more iconic hairstyle, and tweaked the crown a little bit. Tried to make Flame Princess look a little more feminine, and gave her more neck as well. The original intent was to have her look as though she was glancing up at the viewer coquettishly, dunno if that reads properly. I gave Beemo the correct number of speaker holes, and brought marceline's hair down. I tried giving her two fangs but I feel it makes her mouth look too busy. I also darkened up her bite marks a little, hopefully that will make her slightly more recognizeable.