I really like games about possessing different characters with different abilities! I'm glad you made this!
I like the parts on the levels where you have a choice of which target to possess. I'd like to see this more frequently in levels.
A couple of the early levels start too similarly and feel repetitive. I think your game would be better paced if you made the later (and better) levels sooner.
Some of the characters walk very sluggishly. Maybe this is appropriate to the type of creature they are, but it doesn't feel very good to use. I think the game would feel better if all the creatures moved faster, while still keeping the slowest ones the slowest.
I think the jumping would feel better if it were less floaty. The floatiness is especially strange when using the slime, which is particularly sluggish in walking, and feels heavy, but somehow floats in the air during jumps.
It also feels strange that the creatures that jump can jump up much faster than they can walk. I suggest having the jumps for each character to be lower (along with lower obstacles), and walk speeds faster so they both walk and jump speeds/heights feel like they belong to the same character. Try to think about how many times the character's body height it should be able to jump per second, and how many times the character's body length it should be able to jog per second.
Sometimes, your target is something slightly offscreen or entirely offscreen, which feels weird. It would feel better if anything you need to target with your possession ability would be placed not too far away to fit on the same screen as the player.
The snake doesn't feel very cool because the slime can do all the same things and also jump. Maybe take away the slime's climb ability so only the snake has it to make each feel important in its own way.
The text at the bottom of on the title screen is very hard to read.