Mashiro is a young girl with long dark brown hair that is tied in a ponytail with a big white ribbon where it is sometimes depicted like a tail, straightening and curling depending on her mood and large yellow eyes.
Since she is actually grade school-age, her high school uniform is far too large for her as shown by her sleeves which cover her hands and she is also smaller than the average 9-year-old as Kobeni Yonomori does not believe her when she says that she is 9-years-old at first, which seems that being petite may run in her family, as seen with Shirayuki Mitsumine (Mashiro's mother).
Mashiro has a bit of a bratty personality, but she does love Hakuya Mitsumine (Mashiro's older brother) and the many friends close to her. She will often try to act older than she is, by addressing others, such as Kobeni Yonomori, very formally. She also demonstrates a wide use of vocabulary, despite her young age.
Mashiro is seen as intellectual, however is seen to be false and was nothing more than an over-exaggeration from Kobeni when she discovered Mashiro's true age. As such, she came to the conclusion that the only reason Mashiro was allowed in High School was because her level of intellectual was too great. However, it turns out that Mashiro used her powers as a means to successfully pull off being a High School student and as such, her level of intelligence is actually average.
Hakuya is Mashiro's older brother, whom she is very close to. Hakuya almost always knows how to deal with Mashiro. Despite this, she is unable to read his emotions, unlike Kobeni whom reads them with ease. Mashiro also tends to slam the door at unfortunate moments, the force of which knocks over the house Hakuya had been building. Mashiro also has a habit of disturbing Hakuya when he is building famous monuments using match sticks.
Mashiro acts very coldly towards Benio, due to an obsession she has with Mashiro (Most likely explained by her size), much to the younger girl's chagrin. Mashiro will often try to run away from Benio, or will hide behind her elder brother or Kobeni when Benio begins to scare her. However, she will occasionally accept Benio's invitation to the student council room, since she likes the tea and sweet food that Benio offers to her. She is in love with Mashiro and hates her brother.
Mashiro is Kobeni's future sister-in-law. Kobeni is careful about what she cooks due to Mashiro's tastes, but threatens to cook all the things she dislikes when Mashiro goes against her wishes. The pair have a very neutral relationship.
Shirayuki is Mashiro's mother. Mashiro complains to Hakuya about how their mother says things without thinking, as she shocked Kobeni by telling her that the Mitsumine family were not human even though Mashiro and Hakuya had yet to tell her.
- The name Mashiro means "real, genuine" (真) (ma) and "white" (白) (shiro).
- Mashiro's surname Mitsumine means "three" (三) (mitsu) and "peak" (峰) (mine).
- Mashiro often tends to barge into the room whenever Hakuya Mitsumine is making a matchstick house
- Mashiro hates Sundays
- Mashiro likes curry mild while she does not like sticky food.
- Mashiro is afraid of U.F.Os, monsters, Benio Yonomori while she dislikes sticky foods and fermented soybeans.
- However, she does like sweet things and tea.