Cousin Monster is Cookie Monster's toddler 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.
In one Sesame Street 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 a carrot, a cucumber, some cauliflower and an ear of corn. 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.
The puppet was often used as Cookie Monster's Sister, such as in the song "Me Gotta Be Blue". Also, in the song Don't Eat the Pictures (from the 1983 special of the same name), the puppet was used as one of the Cookie Monster Angels. Here, the eyes were slightly remodeled to appear more googly.