Mato is a black-haired school girl with blue eyes. She has short, choppy pig tails and wears a seifuku consisting of a white top with a dark blue collar and a dark blue skirt. She also wears black knee-high socks. She is also seen to have a patch of bandage on her right knee, after obtaining an injury from basketball practice. Many of her belongings, including her clothes, phone charm, and room decorations, have a star motif.
She is described as being 155cm tall, while her "other self", Black★Rock Shooter, must be about 156-161cm.
She is an innocent and naive first year student in middle school. Outwardly, she is outgoing and excitable, but she is very awkward around people she does not know. Internally, she is sensitive and meek, but eventually learns to gain courage.
In her first year of junior high school, Mato comes upon Yomi Takanashi when looking for friends to make and decides to strike a conversation with her. Mato's bluntness and outward personality shocks Yomi initially, but the two become friends eventually. They begin to turn to each other for everything, and Mato eventually shows Yomi a special place which is known as the Meeting Place where one can see a large view, and gives Yomi a cell phone charm with a star (identical to the one she has herself). Additionally, Mato's skill in basketball encourages Yomi to join the volleyball team.
Things change in her second year. The two are put in separate classes, and Mato finds and befriends the basketball team's manager Yuu from her class. Mato initially tries to get both Yuu and Yomi to be together with her at the same time, but ends up spending more time with Yuu. This makes Yomi feel "left out" and causes the rift to increase.
Mato starts to worry about Yomi, but things get serious when Yomi suddenly vanishes, not responding to any calls or texts. Initially Mato believes this means Yomi is avoiding her, but when two men come to her house asking about Yomi's disappearance, Mato breaks into tears and continues halfheartedly searching for Yomi.
Eventually, Mato gets a mysterious text from Yomi with nothing in it. Convinced that this is a clue, Mato runs out of her house (wearing a black jacket with a white star on the back) and rides her bike up to the place she showed Yomi. She finds the phone charm she bought for Yomi and holds it to her chest, finally realizing what Yomi means to her.
A light erupts from the charm and envelops her, transporting her to a strange place filled with light. She meets Black★Rock Shooter there, and reconciles with her "other self," who offers to help Yomi. The two merge and both identify herself as Black★Rock Shooter, allowing both to save Yomi, who has been taken by Dead Master.
In the end, a reconciled Mato and Yomi walk home from school; however, Mato remains unaware that Yuu is in a dark state...
- Mato's surname is sometimes misread as "Kokui" due to the fact that it would be the more common pronunciation of the kanji in normal wording: however, "Kuroi" as a surname is the proper pronunciation.
- The kanji for her surname means black clothes. "Mato," with certain kanji, can mean target (as in target shooting).
- Her father is not mentioned in the OVA. The anime, however, establishes its Mato counterpart as having a father who is away on business.
- The name 黒衣マト (Kuroi Mato) is pronounced the same with 黒い魔都 (Black Devil's Castle).