Louise Marsh is the fiancée of Simon Tremblay. She was mentioned in Sandcastles in the Sand, P.S. I Love You and Bedtime Stories.
Louise Marsh is the subject of an ongoing joke throughout the show, as we see Simon leaving things in his life behind, namely, his relationship with Robin, to be with Louise.
In Bedtime Stories, Robin runs into Simon in a bakery, where he tells her that he and Louise Marsh are getting married. Robin later stole his and Louise's wedding cake, and proceeded to eat the entire thing.