|Appearance||Period 42: Best Friend Chocolate|
Mari is a young girl with short black hair that frames her face reminiscent of a bob with split bangs. She has slightly curved, rounded eyes.
Mari is described as being a talented cook who is very kind and wise. She finds the confusion others have regarding her friendship with Hina to be amusing, and she considers her to be her precious friend. She deeply cares about Hina and being away from her makes her upset, but because of her desire to help her, she endures it. She was also willing to forgive Hina for what happened after being hurt by her, and was very concerned for her. It's shown later in the story that Mari is trusted enough by the teaching staff to portray certain behavior in an attempt to lure Chiyoko to them.
With Valentines Day taking place, Mari and Hina exchange chocolate and admire each others efforts and give some out to some other girls. But after Mari witnesses Hina being picked on by the two girls, she laughs before explaining their bond to them in order to make her feel better.
The next day Mari is startled as she observes Hina behaving weirdly upon finding a present in her shoe locker and suggests she open it. After Hina ran away in fright she chases after her and listens in class when Hina fills her in on what happened the previous day after school. She asks if Hina got a good look at the suspicious woman, and having been overheard by a classmate, he and his friends join them to tell them about Chiyoko-san. Mari finds the story questionable and tells Hina to brush it off since the guys are probably just teasing, and during class she tries to calm Hina down when she finds the re-wrapped present within her desk.
As lunch arrives, Hina is upset when the girls she approach run away from her and Mari happily rejoins her with a jump rope and suggests they go play. Hina hesitantly agrees, asking Mari why she hasn't run away from her like they have and Mari assures her that she will be fine. She already handed the chocolate over, and a patrol was set up circling the school to make sure nothing happens. She also says that they are best friends and she can't stand the thought of being without her.
As she calms down a tearful Hina, they notice a group of alarmed students approaching the teachers office. One of them had just received chocolate, causing them to realize Chiyoko is in the school somewhere, and Mari is quick to jump to Hina's defense after a student begins to blame her. As Mari tries to say she isn't at fault- she is overcome with shock to find Chiyoko standing outside of the nearby window. Angered, Hina tells off Chiyoko and asks Mari to grab the teachers, but she expresses disbelief when Mari suddenly turns her back and runs away with the others.
Once school ends, Mari decides to leave with a few girls in class and ignores the sad and lonely Hina, who can only question her sudden neglect of her. She is taken into the hallway with the teacher as her parents will be picking her up, but as Mari heads down the hallway Hina is overcome with rage as she hears someone voice how much they pity her. This incident has caused everyone to avoid Hina- even her best friend.
Meanwhile, Mari explains sadly that as much as she wants to help Hina she knows she isn't capable of it, and they must let the teachers handle it. She splits up from the other girls and heads down the nearby stairwell when a betrayed Hina trails behind her, then shoves her down the stairs before Mari can even realize this.
After running home Hina takes shelter by a table in her living room, and aware of her parents being gone she kills time by observing her cellphone. But to her alarm she finds a text from Mari that was sent around lunch. Mari explains in the text that as painful as it is, they must take a little time apart so that they can lure out Chiyoko. She and the Teachers have come up with a plan to capture her, and they needed to isolate Hina knowing she probably wouldn't make herself known otherwise. But she asks that Hina remains by an adult so that she will be safe.
As Hina realizes that she has done, Mari is shown to frantically be calling her phone at the hospital. She worries when she doesn't get any sign of response though, and a nurse approaches her to point out that with her injury she should be resting. Mari agrees and hangs up the phone when suddenly the nurse holds up a wrapped present, saying a strange woman dropped it off.