She is slim and of average height for her age. She has straight black hair which is parted to the left side of her face. She has dark colored eyes and normally keeps a cold and angry expression, but quickly tears up or turns red when emotional—in line with her tsundere personality.
She has a harsh personality and low tolerance for those who come to school for reasons besides studying, which distances her from the other students. She also has a strict policy on following the rules, and gets furious when others do not. Her strong personality also defends her well against gossip.
She tries hard in everything she does, be it studying, club activities or playing with the children. Although she is devoted to her studies and does not initially try to get along with her classmates, she feels the strain of being alone and becomes more eager to make friends—being noticeably happy when she first shares contact details with new friends (only having her parents' info before).
She is quick to flip between being cold or angry to upset or embarrassed, often due to her awkwardness with social situations from studying all the time. She loves the daycare children but does not express it well. She adored by Kirin who sees her as an older sister figure despite her gruff behaviour. Despite her anxiousness in unfamiliar situations, she is considerate to others especially Ushimaru, and avoids causing trouble to them.
Inomata appears to be the only daughter of Gorou and Kimie, both of whom uphold principles of strict upbringing with their child. It was revealed that Maria was not allowed to watch television even as a child, and when discussing Kirin's favourite anime, "Petit Pois", she had no idea about the show's characters, though she took a liking to it when Usaida played it on his handphone for her to watch.
She first appears when the daycare children follow Ryuuichi through the school, to which she publicly chides him for his negligence. The next day, she sees Kamitani Shizuka carrying a stack of folders and offers to help. The teacher asks a favour of her, asking Inomata to take her son, Hayato's lunchbox to the daycare. Inomata arrives to hear Ryuuichi trying to comfort a despondent Kotarou and tells him that he has "no reason to be upset" by her comments, but she argues that the reason is obvious—it is due to Ryuuichi's negligence.
Usaida, after reading a message from Shizuka, pretends to have a stomachache and forces Hayato to take him to the nurse's office. Usaida leaves both Inomata and Ryuuichi in charge of the children, not giving Inomata a chance to complain. When Ryuuichi tells her he is fine to care for the children on his own, Inomata refuses the way out, saying she will not be so irresponsible to leave the children all alone.
When the children refuse to approach her, Inomata offers to hold Midori for Ryuuichi, but her loud and angry voice accidentally makes Midori cry and the children attack her, with Taka calling her a "baddie". Asking who he is calling a baddie, she accidentally scares them into submission and they cower behind a table. Ryuuichi tries to make the children apologise, but she tells him not to bother, seeing that he is in a worse and more "pitiful" situation.
But Ryuuichi tells her she is mistaken and that he is truly happy, she starts throwing toys at him and shouting, realizing her own situation is more pitiful with the children fearful of her and her classmates perceived to be hating her because all she can do is study.
She cries openly, frustrated, but is comforted by the children who relate to her pain and cry in sympathy too. She holds them close and exhausts herself crying, accidentally falling asleep with the children and waking up when the kids have arisen and are eating lunch. Embarrassed, Inomata claims that she does not skip class like this while Ryuuichi apologises, telling her she would not wake up after his attempts to do so.After repeatedly meeting Ryuuichi, it becomes clear that she has developed a crush on him (though she does not know it), even going so far as to try to make him Valentine's chocolates.
It is through their mutual affection for Ryuuichi that Ushimaru and herself become friends. Ushimaru later introduces Inomata to her other friends over New Year's.
When the daycare children (along with Ryuuichi and Hayato) invite her to play in the snow and she goes (to the shock of her mother), she accidentally brings along fellow advanced class student, Yagi Tomoya, who had saved her from falling over into the snow and calls the Nezu siblings to play as well.
Ryuuichi invites Ushimaru too and Usaida shows up carrying Midori, then asks Inomata if she has ever made a snowman before, saying she gives off a "vibe" that she has not. Inomata says she did once in elementary school, but admits to having done it alone, inspiring everyone to build their own snowmen for her.
The ridiculousness of everyone's creations makes her laugh, but she questions Usaida's opinion that the one she helped make is equally weird. Ushimaru encourages her to send a picture to their friends and she is characteristically baffled by their response, not understanding the shorthand for laughter and kaomoji.
- Her first name is revealed when Ushimaru refers to her as "Maria-chan".
- She is very particular about the temperature of her baths.
- Ino from her name, Inomata, is a reference to Inoshishi, meaning "wild boar"
- In chapter 21/Episode 7, Kirin sticks a wild boar sticker on her skirt
- In Episode 2 of the anime, Inomata's Eyecatcher depicts her hugging a stuffed boar plush.