- Saki Watanabe: Saki is a childhood friend of Shun as well as his first crush. During the camp when they were paired up for night canoeing, Shun got rid of the waves to let Saki enjoy the stars' reflection on the water, not too long before telling her they should go back and taking her hand in his. After the first timeskip, Shun and Satoru entered a relationship. After he became a karma demon, he distanced himself from everyone, but before disappearing, he met Saki one more time and gave her a collar he made to defend against the Tainted Cats. When Saki managed to find him after he disappeared from school, Shun told her about his condition and confessed that he's always loved Saki before dying. Later he appeared in Saki's dreams as a faceless boy. Saki finally remembered his name and face in Tokyo, and he helped Saki realise the true nature of the Fiend. In the manga, Shun fell in love with Saki due to their common desire to leave the Holy Barrier and despite their mutual feelings, Shun eventually pushed Saki to realize her feelings for Satoru after his death.
- Satoru Asahina: Satoru is a childhood friend of Shun. Initially, Satoru viewed Shun as a rival to compete with. After the timeskip, he became his boyfriend. Eventually, Shun broke up with Satoru, stating that there is no sense for them to pretend being lovers any longer and that he wanted to be by himself for a while, leaving Satoru heartbroken. This was done, however, in order for him not to harm Satoru any longer due to the fact that he was turning into a karma demon.
- Maria Akizuki: Maria is a childhood friend of Shun and a member of Group 1. Maria, while reluctant to search for him at first, eventually became one of the two to witness the Tainted Cats being sent to kill him.
- Mamoru Itou: Mamoru is a member of Group 1. The least enthusiastic to search for Shun and to investigate the truth, he is the other one to witness the Tainted Cats being sent to kill Shun. Mamoru is shown to be the most distant with Shun as they had not attended the same school and were not acquainted in childhood.
- Subaru: Subaru is Shun's pet bulldog. Shun took care of Subaru like a surrogate mother as Subaru's mother died in childbirth and the rest of the litter did not survive. Since Subaru could not walk very quickly and got tired easily, Shun could not bring him everywhere he went, but once in a while they could be seen taking a walk together. After Shun's disappearance, Subaru follows him as his faithful companion, despite Shun trying to leave him behind several times. When one of the Tainted Cats comes to dispose of Shun, Subaru attacked the cat to defend his owner, and was killed during his attack. This triggered and finalized Shun's decision to end everything with his death.
- Shisei Kaburagi: Shun was thought of as Shisei's successor due to the fact that he was the most talented out of all of his generation. However, during his visit at the Sage Academy, he was the first to realize Shun's transformation into a karma demon and the one to raise the alarm.
- Reiko Amano: Reiko is a childhood friend of Shun and a member of Group 1. Reiko is seen playing capture-the-flag with the other children in their childhood.