Katherine "Kate" Monster is a character from the Tony-award winning musical Avenue Q. She was originally performed by Stephanie D'Abruzzo, who was nominated for a Tony Award for her performance. Kate Monster was performed in later Broadway casts by Mary Faber, Sarah Stiles, Carey Anderson, and Anika Larsen. She is played by Julie Atherton in the UK productions, where she is given a very nasal voice.

According to the West End production's website, she is 23 years old. Kate is a kindergarten teaching assistant. She is an assistant kindergarten teacher who works for Mrs. Thistletwat. She would like to open a school for monsters someday though. Even though she's performed by D'Abruzzo in the original Broadway show, the character's voice sounds like that of Prairie Dawn, who was performed by Fran Brill until her retirement; now Stephanie performs Prairie using her Kate Monster voice.

Kate Monster is Princeton's love interest, and she likes him as well. In the show they have a fight, so Princeton suggests that they should just be friends. While Lucy T. Slut is in a coma, Princeton and Kate try to patch things up.

In an advertisement for the show, she was described as a "Slightly overweight puppet seeking [a] soulmate."

The Kate Monster performer also performs Lucy T. Slut.

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