Masaaki Azumi serves as a caretaker and researcher at the Enokura Aquarium, tasked with overseeing Umi and Sora—two enigmatic boys with aquatic abilities tied to supernatural disturbances in marine ecosystems. Balancing scientific rigor with a protective instinct toward the pair, his approach blends pragmatism with compassion. His professional history intertwines with his estranged wife Kanako, formerly his collaborator at the aquarium until their separation following their daughter Ruka’s birth. Though their relationship remains strained, their shared concern for Ruka and lingering professional ties form fragile bridges between them.

Masaaki’s bond with Ruka is marked by quiet distance, rooted in childhood visits to the aquarium that sparked her fascination with marine life. These memories linger in his current efforts to navigate parenthood amidst his duties, though their interactions grow sparse as she matures. His composed demeanor anchors both his work challenges and personal struggles, even as unresolved questions about his deeper motivations and internal tensions remain unexplored.