Mildred "Millie" Burtonburgeris is Coop's little sister. She is very manipulative with her dad. She spoils Mr. Kat and sometimes teases Coop. Her regular clothes consist of a sea green shirt, blue dress, red glasses, red socks and a reddish-pink bow in her hair which is almost always in a wavy ponytail. She also wears brown shoes. Millie is enemies with Phoebe, who heavily crushes on Coop. She is a "Greeny Girl" (Bootsvillie's girl scouts are called Greeny Girls and the boy scouts are Lobster Lads). She is the "owner" of Kat and calls him by Mr. Kat. Her real first name is Mildred. Millie uses her screams when ever something doesn't go her way, proving she is a spoiled girl. Millie will frame Coop when she does something that would get her cat in trouble, even when Burt caught Kat fighting Coop. Once, Millie still tried to frame Coop.

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