Doi is a male classmate of Shuichi. Doi is very self-confident in himself and his opinions, and this has lead him to, at times, bully other students. In middle school, Doi would often times bully Shuichi for their feminine appearance. But in high school, although being a 'cool kid' with the other students, and still displaying the arrogance and privilege of that status, he seems to slowly have a change of heart. After meeting Yuki, a transgender woman, his admiration of her beauty causes a 'tipping point' in his social attitude to others. His admiration and gradual approval of Shuichi presenting as a girl seems to temper his overbearing attitude, and he tries to befriend Shuichi, apologizing for his past conduct. He even begins encouraging Shuichi to come to school dressed as a girl. Later, during the fall-out of Shuichi's presenting as their correct gender, Doi snubs Shuichi, showing how quick he can be with emotional swings and trauma. It seems that Doi has a hard time dealing with what kind of person he wants to be, and be seen as.
Much later, Doi begins to consider cross-dressing as a teenage girl himself. Doi is not a transgender person, but seems to have a fascination with gender concerns and dressing as the opposite gender.
Doi begins attending the same all-boys high school as Makoto and Shuichi, while Yoshino and Saori begin attending another high school where school uniforms are not required. In middle school, Doi used to bully Shuichi in the most cruel and demeaning way because of Shuichi's feminine appearance and demeanor. To compensate, Shuichi would merely try and avoid any contact with Doi. But in high school, Doi begins to take an interest in Shuichi when Doi sees them with an older woman and wants to know how Shuichi knows her. Shuichi merely tells Doi that the woman is an adult friend. Doi eventually meets Yuki, and is very impressed by her beauty and demeanor. So when Doi and Shuichi next see each other at high school, Doi tries to befriend Shuichi and tells them that he is sorry for the bullying when they were younger. Doi's interest is further piqued, when he learns that Shuichi is a transgender teen who wishes to transition to a girl, and that Yuki is a transgender woman who has already transitioned to an adult female. This sparks an interest when he sees Shuichi dressed in a girl's pretty dress, prompting Doi to admire how attractive and pleasant Shuichi is presenting as their authentic self of a teen girl. He is able to convince Shuichi to wear female attire to school. Shuichi does so, but with less than desirable results.
Later, Shuichi takes the opportunity to 'call out, tell off, and take down' Doi verbally, later expressing wonderment that they found it in themselves to even be possible to confront Doi in such a fashion. However, when Shuichi is having trouble with writing the script for the gender non-conforming play to be shown at the school's cultural festival, Shuichi asks Dio for help. Despite the animosity between them, they put their differences aside, and work together to make the script and play a huge success. Doi is cast in a role for the play where he portrays a teenage girl, and greatly enjoys wearing female attire, and acting the part of a teenage girl, on stage.
After graduating from high school, Shuichi moves out and goes to the same college as Doi. Doi and Shuichi continue their uneasy, changeable, and often times rocky friendship. Shuichi continues to write a novel, which is given the title The Boy Who's a Girl (ぼくは、おんなのこ Boku wa, Onna no Ko).