From the Super Mario Wiki
The Flip-Flop Folk are the entities that inhabit Flipside, Flopside, Yold Town, and Gloam Valley in the game Super Paper Mario. What manner of creatures they are is completely unknown and never revealed, although they share many similarities to humans. Almost every Flipside character has a counterpart in Flopside; the counterpart having the opposite personality. Their bodies come in yellow, purple, and blue colors, are squarish in shape, and some of them seem to be lacking arms. While the Flipside residents have square noses, the Flopside residents have pointed noses, and the residents of Yold Town and Gloam Valley do not have noses at all.
List of Flip-Flop Folk