Mai is a 38 year old woman, who nonetheless retains a youthful appearance and has a slim build. She keeps her light hair in a bob cut.
Mai wears a plain shirt under a tartan-patterned mini-dress which is decorated with several buttons put her and there on it. The dress is notably cut low enough to entirely reveal Mai's torso and her shirt.
Mai is a peaceable and polite woman, usually keeping a polite and unassuming behavior. She notably doesn't react to Rohan Kishibe's constant bluntness and is even understanding of his refusal to help make Mao a normal child.
Mai is marked by the death of her former fiancé, and has not been able to forget her feelings toward him, being incapable of loving another man. However, Mai longed for a child and accepted to use a sperm bank to bear her daughter, Mao. Despite Mao's strange appearance and behavior, Mai loves her daughter and is thankful to have her. Although she is peaceable, if she thinks Mao is in danger, Mai will immediately jump to her rescue.
15 years before the events of Episode 8, Mai Katahira was engaged and freshly married, but then suffered a car accident in which she lost her husband. During the next years, Mai wasn't able to move on from her husband, dating several men but inevitably breaking up. However, Mai also underwent artificial insemination and had Mao, which she loved despite her difformities.
As Mao grew, Mai began to worry that she would bullied for her appearance and got into contact with Rohan Kishibe through Yukako Yamagishi and asked Rohan if he could cure her. However, Rohan wasn't interested and when he turned Mao into a book, claimed that she was ultimately fine the way she was. Nonetheless, Mai learned that the biological father lived in Yagatama and was 30 by the time he donated his sperm. Thus, she could investigate the cause of Mao's quirks.
Later, Mai incidentally saw a man who both fit the clues and reminded her strangely of her late husband. Following him up to the park Mao was playing, Mai was then victim of a prank: Mao shed her skin and disguises a friend as herself, making Mai believe she was being kidnapped by the same man. The misunderstanding being cleared, Mai then became acquainted with the man, named Obanazawa, and came to like him immediately.
Less than 3 months afterward, Mai is stated to have married Obanazawa.