Role in the show
Ursula is never actually shown (except once in a drawing by Stumpy), but she is mentioned several times on the show, and is occasionally relevant to the episode's plot. She is Stumpy's girlfriend, who he met on Fakebook. In "Let's Play Party", she was supposed to come to the Valentine's Day party but she never showed up. In "Let's Play Hot - Cold", Stumpy was looking for ways to impress her. In "Let's Play Musical Comedy", Stumpy tried to write a song for her. In "Let's Play Astronauts", Stumpy went in search of a star to name after her, kicking off the episode's plot. Ursula was one of the main plot motivators of "Let's Play the Very Special Episode", despite not appearing onscreen. Pretty does not believe that Ursula actually exists, but it's possible that she does, as Stumpy is able to communicate with her via internet or phone.
Ursula is never shown onscreen physically, but one episode has Stumpy do a drawing of her. The drawing is rather crude, so all that the audience can make out of her actual appearance is that she has blonde hair. However, as Kaeloo points out, Stumpy may have drawn her inaccurately.