Mildred "Millie" Burtonburger is the tritagonist of the show, Kid vs. Kat. She is voiced by Kathleen Barr, who also voiced Trixie and Queen Chrysalis in My Little Pony: Friendship is Magic, and Mrs. Test in Johnny Test.
Coop's very annoying 8-year-old (turns 9 in Season 2 and would turn 10 in Season 3 pending season) little sister and Kat's owner. Her father is Burt Burtonburger. Millie is always defending Kat, no matter what he does. She loves to play dressup and tea party with him, and she spoils him on a regular basis. Despite her cuteness and beauty, she is the typical annoying younger sibling to Coop, but she does seem to appreciate when he hangs out with her, willing or forced. She loves Mr. Kat.