Mymble is Little My's older sister and Snufkin's half-sister, sharing the same mother. Their mother, also named Mymble, practices a very liberal parenting style, allowing her numerous children freedom without strict discipline, which means Mymble often doesn't know the whereabouts or exact number of her siblings.

Frequently tasked with caring for her siblings during her youth, Mymble found the responsibility overwhelming and unsuited to her carefree nature. She now prefers an independent life in Moominvalley, prioritizing personal freedom and living in the moment.

Possessing a highly imaginative personality, Mymble frequently invents fantastical stories, like claiming she was born in a clam or that an uncle keeps white mice in his beard. These tales entertain herself and others, though they sometimes involve dramatic falsehoods, such as pretending to be ill as punishment for misbehavior.

Physically, Mymble is depicted as light and thin with long legs, contrasting with her rounder mother and smaller sister, Little My. She wears her long red hair in a distinctive "haughty mymble hairdo," described as glossy and tight, resembling "a soft-reddish yellow like a little onion." She takes great pride in her hair, brushing it frequently and washing it in rainwater every fifth day.

Mymble often longs for love but tends to pursue unsuitable partners, like pirates or winter sports enthusiasts. Despite frequent disappointments, she recovers quickly and maintains a cheerful outlook. Her social interactions are characterized by a carefree attitude others may initially find charming but eventually perceive as tiresome or bothersome due to her lack of consideration for responsibilities or consequences.

She first appears in the narrative when Moominpappa and his friends encounter her during his youth. She bonds with the Joxter during this meeting, though he is only the father of Snufkin among her many siblings. In later adaptations, she visits the Moomin family directly, bringing her children along. Her parenting style during visits involves minimal intervention, like pouring water over misbehaving children, and she shows little concern about leaving Little My with the Moomins permanently due to her numerous offspring.

Mymble demonstrates artistic inclinations, being a skilled dancer who takes readily to acting. When the Moomin family stages a play, she insists her character must be eaten by a lion at the story's conclusion, showcasing her flair for dramatic narrative choices.

Her philosophical outlook emphasizes simple pleasures and personal freedom. She expresses enjoyment in activities like lying on a bridge to watch water flow, wading through swamps in red boots, or listening to rain on a roof, valuing doing "just what one feels like" and rejecting external pressures.

Mymble's design evolved across adaptations. Original book illustrations featured her with stylized ears and a tail, while later comic and anime versions gave her a more humanoid appearance without these features, resulting in some adaptations depicting both design interpretations as separate characters.

Her independence is further illustrated by a period living on a haunted island with Moominpappa and his friends, including Hodgkins, the Joxter, and the Muddler, underscoring her adaptability and comfort with unconventional living situations.