Mashiro Mitsumine is a nine-year-old girl with long dark brown hair, typically secured in a ponytail by a large white ribbon. This ribbon often resembles a tail and subtly shifts with her emotions. Her large yellow eyes and petite frame contribute to an appearance younger than her years, a trait inherited from her mother. Despite her grade school age, she attends high school by employing a hypnosis-like ability to blend seamlessly among older students. This creates an illusion of exceptional intelligence, though her academic performance remains average for her actual age. Her high school uniform is noticeably oversized, the sleeves often covering her hands.

She exhibits a bratty demeanor yet holds genuine affection for her older brother, Hakuya Mitsumine, and close friends. Striving to appear mature, Mashiro frequently addresses others formally, particularly Kobeni Yonomori, her future sister-in-law, and utilizes an advanced vocabulary that clashes with her youth. Her interactions with Kobeni are generally neutral; Kobeni adapts meals to suit Mashiro's food preferences and employs culinary threats to influence her behavior. Mashiro harbors significant fear toward Benio Yonomori, Kobeni's older sister, often hiding behind Hakuya or Kobeni to escape Benio's intense focus. Despite this fear, she occasionally visits Benio's student council room to accept offered sweets and tea. She also fears UFOs, monsters, and unexplained phenomena, though toy models of such entities later capture her interest.

Her bond with Hakuya is deeply familial; she relies on his understanding but struggles to interpret his emotions, unlike Kobeni. Mashiro habitually disrupts Hakuya's matchstick model-building hobby, often barging into rooms and accidentally destroying his intricate creations. Regarding her mother, Shirayuki Mitsumine, Mashiro expresses frustration over Shirayuki's tendency to reveal sensitive information without forethought, such as the family's non-human heritage.

Mashiro belongs to the Inugami lineage, a supernatural species explaining her crowd-blending power. Her background includes a pivotal incident where Hakuya saved Kobeni’s life by sharing half of his Inugami power, though Mashiro’s own abilities remain distinct. She dislikes Sundays, sticky foods, and fermented soybeans, while favoring mild curry, tea, and sweet treats. Her name translates to "real, genuine" (真, *ma*) and "white" (白, *shiro*), with her surname Mitsumine meaning "three peaks" (三峰).