Technically a fan-game for the Gamougg series by John_Shrekinson.
WASD and Arrow keys to move
Numbers: 1-3 for costumes (0 for off costume) Hooray
JUMP ON SLIME MAN TO KILL HIM
Gamougg gets teleported to Threa a planet that looks just like Earth. (Put the word Threa backwards and it says Earth), but the Unrelateable Boring Invincible Slime-man lives there and likes to eat skeletons, so now Gamougg has to defeat him in a battle, even if he is Invincible