From the Super Mario Wiki
Slim is a resident of Flopside in Super Paper Mario. Throughout the game, he is in love with many different women, such as Tinga and Merlee. During the prologue before Chapter 7, he is in love with Nastasia. After the end of the game, he says that he met Nastasia, but her heart still belonged to Count Bleck.
In the NTSC version, his counterpart in Flipside is Chap, who, oddly, has no similarities with him in name or personality. In the PAL version, his counterpart is called "Glim" (a much more similar name).