- Marry Kozakura: Seto first met Marry while wandering around the forest and stumbling upon her house, Marry was afraid that she would turn him into stone. However, he misunderstood what she meant and thought she was shy, and put his headphones on her to comfort her. He is Marry's closest friend. There is specific evidence that those two like each other, however, it is not said to be canon. Seto called her cute on accident when a deer and a hamster called her a monster.  In one of the Anthologies, Mary asked herself if Seto was her 'prince' when the two were talking about books/manga. Also, when Mary cooked something spicy for everyone - It was actually for only Hibiya to man up - Seto was the first to eat it because Mary said it was filled with love in one of the 'Spicy' anthology.
- Shuuya Kano: Kano and Seto grew up in the same orphanage together and are childhood friends. The two shared an extremely close relationship, especially as children. Kano was constantly annoyed by Seto's crying fits, as he used to cry all the time. They both expressed concerned over Kido's dispappearances and would frequently talk to each other telepathically. They both called each other by first name until Ayano's death. In the fifth novel and anime Seto goes looking for Kano. He comforts Kano when he breaks down, using is ability to read his mind. They call each other by their first name for the first time in years. Seto considers Kano a brother, instead of a childhood friend.
- Tsubomi Kido: Seto and Kido were put into the same orphanage and were adopted together with Kano by the Tateyama family. In the fifth novel, Kido would threaten to harm Seto at the orphange if he continued to address her with keigo and their was always a sense of conflict between the two. Sometime after Ayaka's death, Kido explains Seto that she didn't like Seto's polite speech was because her previous family made backhand comments using it so her policy was "Friends don't use keigo". The two make amends and Seto adapts his speech pattern into a different variation. They are good friends, and now live together in an apartment.
- Ayano Tateyama: Ayano was told by her mother to be a "big sister" for Kido, Kano, and Seto. She was the one taking care of them in the orphanage. Ayano followed her wish and started to become friends with the three of them, what turned out to be difficult, as they thought of themselves as "monsters", due to their eye abilities. Eventually, Ayano came up with the plan to create a secret club of heroes and gave them their hoodies as a present.
- Kenjirou Tateyama: The adoptive father of Seto, Kido and Kano.
- Ayaka Tateyama: The adoptive mother of Seto, Kido and Kano. He along with Kido and Kano, were crying while holding a funeral for her. He makes so distraught of her death that he runs into the forest and goes missing.
- Ene: He mistook Ene for being Shintaro's girlfriend from an online dating site.
- Momo Kisaragi: He thinks of Momo as a very good and polite girl, as shown from the amusement park chapter. He never thought that Momo was an idol.