From the Super Mario Wiki
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Lucy is a resident of the town of Flopside in Super Paper Mario. According to Tippi, she is independent, makes her own rules, and has a noble heart, although is often misunderstood. Like most Flopsiders, she has a look-alike in Flipside, in this case being Muffy. Her favorite word is "drama", possibly indicating that she loves to gossip.
Strangely, in the PAL version, this girl is called Lacy; while her Flipside counterpart is now called Lucy instead of Muffy. However, the scores in the Flipside Arcade are still listed under MFFY, which is probably an oversight on the developers' part.