- Mamoru was an only child. His mother died of typhoid fever when he was young, and since meeting Maria he seems to view her as a surrogate, since they both had red hair, a rare trait in Kamisu 66. According to Satoru, red hair was never an Asian trait, so Mamoru's mother and Maria's possibly mutual ancestors possibly came from a distant country many generations ago.
- Unlike the rest of Group One, he attended Friendship School instead of Harmony School, and only met Saki and the rest at Sage Academy.
- When the Ground Spider colony attacked them, he was successfully able to hide from them and reunite with the rest of Group 1 at the place where they hid the canoes. Later, he was able to regain his Cantus.
- At the age of 14, when all his peers were engaged in homosexual relationships, Mamoru was not, despite having some boys showing interest in him. He spent time with Maria regularly under the pretext of drawing her, as heterosexual interactions were discouraged by the Board of Education. After Shun's disappearance, he was the most reluctant to search for him due to his aversion to breaking the rules, but eventually accompanied Maria into the inner court of Sage Academy and witnessed the Tainted Cats being sent out.
- During the duty pair nominations, he successfully paired up with Maria. Immediately after, when the group attempted to uncover the truth behind their missing group members, he suffered a breakdown due to his aversion to breaking the rules, causing Maria, Saki, and Satoru to conceal their discussions from him.
- However, his aversion to rule-breaking did not save him from being marked for disposal, as he was eventually attacked by a Tainted Cat. Understanding that the Board of Education planned to dispose of him, he left the village. However, he fell off a cliff and was saved by Squonk while passed out. When the rest of the group caught up with him, upon hearing his side of the story, Maria promised him that they will never be apart again, subsequently joining him and leaving the others behind. 
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