In the game Nuclear Throne, what character is only usable in a play through with Melting?

In the game Nuclear Throne, the character that is only usable in a playthrough with Melting is Skeleton. This is because Skeleton is an unlockable character that becomes available once Melting's "Ultra Mutation" is obtained. The player needs to have Melting's passive ability called "Remove Brains" in order to unlock Skeleton, as they both have a thematic connection to each other. Skeleton's unique play style revolves around their ability to survive with only one health point, making them a challenging and rewarding character to master in the game.