It centers around fifteen year old Sawako Yarimizu, who, after being caught in a sexual position with her boyfriend in a classroom by another student is bullied, harassed, and excluded by the other students. She leaves the school, staying home in depression. To keep herself busy while cheering herself up, Sawako continues a manga she started when she was younger. She tells her mother she is studying, while in fact she submits her manga to a magazine called Shōjo Friends. The magazine calls her story too adult and wants her to write more about herself, so she writes one hundred pages about how she was tormented and ridiculed at school. At the end of the story, Sawako goes to graduation with her haircut and a new attitude. She tears her diploma and continues writing manga, after realizing that her new story won an award. The manga is called Vitamin because Sawako finds that expresiing her feelings through manga makes her feel better and more confident. She calls it her Vitamin.
- Sawako Yarimizu- Sawako is the main character, a fifteen year old girl with a normal life until seen in a compromising position with her boyfriend. Her attitude changes drastically from being a sensitive sad person to a much tougher and confident one. She learns that she must face the students and fight back rather than hide at her home.
- Kouta- Sawako's boyfriend. Though he forced Sawako, he denies doing anything with her, making Sawako look like a slut. Sawako was his girlfriend since the end of the second year of middle school. He promises her that he would stay by her side, and after lying to the other students, Sawako breaks up with him.
- Aki- One of Sawako's four friends. She and the other three mistreat Sawako most once they find out about what she did.
- Ryo- One of Sawako's four friends. She and the other three bullied Sawako in the locker room, drawing a panda on her chest.
- Megumi- One of Sawako's four friends. She and the other three harassed Sawako in the bathroom, flushing her head and cellphone down the toilet.
- Yoko- One of Sawako's four friends. After Sawako said she would report to the school board if they did anything again, she and the other three confess to the improved, more confident Sawako that they were jealous that she had a boyfriend in hopes of avoiding trouble. During graduation, they asked Sawako to sign their notebooks. She ended up throwing them out the window, unforgiving her old friends.
- The entire class of 3-2 (Sawako's classmates) wrote a letter to Sawako in Vol. 1 Ch. 2, convincing her that they wanted her back at school, however when she returned, they laughed, saying that the letter was fake and that many of them couldn't believe she actually came. Her four friends plotted the note.
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