Cousin Monster is Cookie Monster's little cousin. She's blue and furry, and has big googly eyes, just like her older cousin. She's still learning to talk, but she has a precocious ability for spelling. She is very affectionate with her cousin, who thinks she's very smart. She wears a red bow with white polka dots in her curly, light blue hair. She loves vegetables, but she has no taste for cookies.
In her debut sketch where the two have a run-in with Ernie, they learn she prefers eating healthy foods over cookies. In this sketch, she's called Baby Monster, both by her cousin, and by Ernie, and was performed by Bob Payne.
In another sketch, Cookie Monster asks his cousin to find something in the kitchen that starts with C. He's hoping that she'll bring him some cookies, but instead she finds some vegetables. He insists that there's something else that's special and wonderful in the kitchen that starts with C. She realizes that her own name starts with C, and he's forced to concede that she is indeed special and wonderful. Here, Cousin Monster was performed by Jerry Nelson.
She also appears in the large crowd of Muppets in the "Rainbow Connection" finale in The Muppet Movie.