She is transparent mucus of a snail of a bluish hue that resembles a grown woman.
Role in episode
Before Gary, Spot, and all the other animals decided to escape the shelter, Gary realized that Marvin is lonely and he felt bad for him, so Gary creates a slimy girlfriend for him and the other animals set up a date for them. After having a long time of fun, they get married at the animal shelter. Marvin thanks Gary and Spot and all the other animals by setting them all free.