Mamoru Yotsuya is the deceased husband of Touko Yotsuya and father of Miko and Kyousuke Yotsuya. His sudden death occurred approximately one year before the main narrative, leaving unresolved conflict with his daughter Miko. This stemmed from an argument after he ate her cherished pudding without permission, preventing reconciliation and becoming a source of lingering regret for Miko.

He manifests as an ordinary middle-aged spirit with visible forehead wrinkles, wearing glasses that conceal his eyes. Unlike many other ghosts, he exhibits no physical mutations or deformities, maintaining a fully human-like form. This normal appearance concealed his spectral nature until Miko offered pudding at the family butsudan altar.

His personality reflects a responsible and caring family man. Despite being a ghost, he continues interacting with his family in their home, showing concern for his wife and children. He speaks kindly to his children, though only Miko perceives him while Kyousuke remains unaware. His behavior establishes him as a peaceful spirit without harmful intentions.

A significant development occurs when a malevolent pudding-obsessed spirit threatens Miko. Her act of placing pudding as an offering at the altar allows Mamoru's spirit to apologize for eating her pudding, enabling their reconciliation across the boundary between life and death and resolving Miko's guilt.

The etymology of his name reflects symbolic connections: "Mamoru" combines the kanji for "real/genuine" (真) and "protect/guard" (守), while "Yotsuya" means "four valleys" (四谷).

Mamoru appears exclusively in Chapter 9 of the manga and its corresponding anime adaptation, Episode 4. His narrative is confined to this reconciliation arc within the core series.