Kobato Naruse is the grandfather of Shiroha Naruse and a respected elder on the island of Torishirojima. He lives with Shiroha in a secluded family home and is recognized as an expert in the island's traditional water martial arts, known as the Underwater Interval Technique. This discipline is rooted in his extensive military background, a past that has left his body covered in scars.

He possesses a strict, serious, and at times intimidating personality that leads others to view him as a fearsome figure. Despite his advanced age, he maintains a powerful physique and considerable athletic ability. His outward harshness masks deep compassion and worry for his granddaughter, and he is troubled by her tendency to isolate herself. His primary motivations are protecting Shiroha and guiding her through the traditions of their family, including the Natsudori ritual, though his approach is often marked by tough love. He extends his protective nature to others on the island as well, acting as a guardian for characters such as Nanami.

His relationship with Shiroha is at the core of his role in the story, defined by his severe methods and his underlying care for her wellbeing. He is aware of the difficult rumors surrounding her and wishes he could help her integrate more with others. Despite his hardened exterior from military service, one of the most telling aspects of his character is his enduring respect for his late wife, whose scolding he once considered more formidable than any battle wound. His deep knowledge of island traditions, his formidable fighting skills, and his fierce, unwavering protectiveness establish him as a key pillar within his family and the community.