From the Super Mario Wiki
Muffy is a teenager resident from Flipside in Super Paper Mario. She is a trendy, care-free and optimistic girl. Her motto, "Life should be short and sweet, babe!" greatly supports this. After the player defeats the final boss, she says when the world was ending, she spent all of her hard earned coins at the Flipside Arcade, and while she did rack up a number of high scores (300,000 in Forget Me Not, 400,000 in Tilt Island, and 500,000 in Hammer Whacker), she was left broke. She, however, shrugs it off and simply says "Easy come easy go!" Muffy then desires to be a Flipside tour guide, and promises that she'll be the best one ever.
Muffy's Flopside counterpart is Lucy. However, in the PAL region, Muffy herself is named "Lucy", while her counterpart is called Lacy. Strangely, despite her name change, the Flipside Arcade still uses "MFFY" as the name in the high scores.