From the Super Mario Wiki
Kerrie is a purple-skinned lady living on Flopside's first floor during the events of Super Paper Mario. She used to be a well-known singer and indeed her singing is quite beautiful, but she is very unlucky and has had no success. Her sadness only contributes to her songs, which she is more than happy to sing for Mario. At one point, Kerrie sings that her boyfriend's true love is the arcade, possibly indicating that her boyfriend is Mort (or Hort). Her Flipside counterpart is Cherie.