- Maria Akizuki: Maria is a fellow member of Group 1, and the object of his devotion. He oftens made sand portraits of Maria using his Cantus to manipulate sand, as a pretext for spending time with her. In the manga, he told her that he had fell in love with her when he entered Sage Academy. Maria became his duty pair partner and left the town with him when he ran away. After the second timeskip, it is revealed that they had a child (son in the novel, daughter in the anime and manga) before both of them were killed by Queerats.
- Saki Watanabe: Saki is a fellow member of Group 1. Although she became Maria's girlfriend after the first timeskip, Mamoru hardly showed any signs of jealousy towards her.
- Shun Aonuma: Shun is a fellow member of Group 1. When the memories of the group were altered to replace Shun with Ryou, they talked about Ryou's house being different from the one they remembered playing at, and Mamoru mentioned that he has never been to there, implying that him and Shun were not as close with each other as the rest of the group.
- Satoru Asahina: Satoru is a fellow member of Group 1.
- Reiko Amano: Reiko is a fellow member of Group 1. After her disappearance, he was as concerned as the rest of the group about her whereabouts. When the memories of the group were altered, Mamoru wass the first one to remember Reiko as a girl they used to know when they first started attending Sage Academy, but he coult not recall her name either.
- Ryou Inaba: After Shun's disappearance, Mamoru believed that Ryou had always been with them since starting at Sage Academy just like the others, due to the manipulation of their memories.