Mimmy is the identical twin sister of Hello Kitty, created by Sanrio in 1974 soon after Hello Kitty's debut. She appears as an anthropomorphic white cat, usually lacking a visible mouth, though some animated shows depict her with one. Her defining trait is a yellow bow worn on her right ear, setting her apart from Hello Kitty's red left-ear bow. She typically wears orange or yellow overalls over a yellow shirt, though early designs featured green overalls; rare depictions show her in blue overalls matching Hello Kitty's standard outfit.

She lives in a London suburb with her family: parents George and Mary White, grandparents Anthony and Margaret, and her sister. Mimmy's personality is shyer, more polite, thoughtful, and girlish than Hello Kitty's. She enjoys sewing, dreams of marriage, and excels at baking, a skill surpassing her sister's. Despite their closeness, conflicts arise, like quarrels over small matters or misunderstandings, such as a baking incident where Hello Kitty ruined Mimmy's attempt to befriend Robert.

Mimmy appears in multiple episodes of "Hello Kitty's Animation Theater," starring alongside Hello Kitty in "The Story of King Midas" and featuring with Hello Kitty and Dear Daniel in "The Three Little Pigs," her roles reflecting her established personality. She also features in the OVA "The Wonderful Sisters," exploring her baking and relationships, and is a central character in "Hello Kitty's Paradise." Cameos occur in animations like "Bunny Maloney." In video games, she often takes secondary roles, such as a second-player option in "Hello Kitty Island Adventure."

Though usually timid, she exhibits moments of unexpected bravery. Cultural depictions sometimes place her in American or Japanese settings, participating in traditions like Thanksgiving or wearing kimonos, and English dubs often give her an American accent. Sanrio confirms her full name as Mimmy White. Her yellow bow, like Hello Kitty's red one, is implied to be a gift from their mother.