He used to write a novel called "Sora ni Niteiru" (Looks Alike to the Sky) using penname Miu Inoue when he was in junior high school and sent to a magazine. The novel was received an award and too famous that it became drama and movie. The name was very feminine that people thought it was written by a mysterious girl.
There was an accident happened to a girl he likes (Miu Asakura) that caused him to have difficulty in breathing so he closed himself until he met Touko in high school, who was reading and eating what she read, so she forced him to join literature club.
Even though Kotobuki hinted the time when they met once, he didn't have a clue until "Angel" book because he didn't see her face at all! He gradually has feelings for Touko and both feeling and responsibility for Nanase in later of Bungaku Shoujo Series. He and Miu reunited, and he learned her real self in "Parumiere (パルミエーレ)" book.
After what happened in "Romantie" book, he had to choose Touko over Nanase, who he was in relation with, and showed what he learned to "imagine" like Touko (basically doing detective thing), and guesstimated everything about Touko. His "imagination" shocked Hinosaka at the first time in 1st book (Hatsukoi) of Minarai series.
After Touko graduated (in Minarai), he became the president of the club and feel sad.
According to first main chapter of Minarai, he wrote a novel confessing that he loves Touko. The girls in Minarai series said that he became mature. Nano Hinosaka, the new main character in Minarai series falls in love with him.
He shows very different personality in Minarai series because he somehow felt that Hinosaka overlapped with Touko and got afraid about his change in feeling (like what he did to Nanase). He is a weak person in feelings that he tends to change who he likes without noticing and get attracted by different girls. Therefore, he hurts girls' feelings without him noticing. However, the feeling to Touko tends to be true, and grows up while taking care of Hinosaka.
According to 6 Year After Episode (6 years after Touko's graduation), he published the novel confessing about Touko and published as Miu Inoue's second novel. He rejected every request to be made as drama or movie. The title is called "Bungaku Shoujo (Literature Girl)"
Bungaku Shoujo Series is in his point of view, while Bungaku Shoujo: Minarai Series is in Nano Hinosaka's point of view.