Ikaruga Natsuki is the seventeen-year-old protagonist of the story. He is a former student of a prestigious academy in a large city, an institution meant for only the most gifted pupils, from which he was eventually forced out. Following this, he returns to live in the small coastal town where his late grandmother once lived, a place now partially swallowed by the rising sea.

Natsuki has a slender build, pale skin, and short, straight black hair with spiky bangs. His blue eyes are narrow, and he often carries a walking stick to assist with his mobility. His appearance is typically casual, consisting of simple T-shirts, formal shirts, and trousers, and he is seldom seen without a cane due to his disability.

His personality is defined by a cynical and pragmatic outlook, a mask that conceals a more cowardly and vulnerable interior. Having endured significant loss and hardship, he initially conducts himself as cold and self-reliant, insisting he needs no one's help. He is easily agitated by unexpected or surprising situations, and he frequently suffers from terrifying nightmares that relive the traumatic accident of his past. One of his most defining traits is a severe psychological phobia: he is stricken with terror when forced into dark, silent spaces, a direct consequence of his childhood trauma.

Natsuki's motivations are rooted in his past and his practical needs. As a child, he lost both his mother and his own leg in a single accident. The artificial leg he uses is not advanced, and his primary driving goal is to obtain enough money to purchase a high-quality cybernetic prosthetic. This desperate need for funds leads him to accept a plan from a debt collector named Catherine to salvage a rumored hidden treasure from his grandmother's sunken laboratory beneath the sea.

In the story, Natsuki serves as the anchor to the human world, a young man whose dreams for the future have been eroded by tragedy and a society that is itself in a state of paralysis due to environmental collapse. His role evolves from a solitary figure obsessed with financial salvation to a more socially engaged person. His initial plan is to sell the strange girl, Atri, whom he discovers in the underwater lab, but he grows increasingly reluctant to part with her as she begins to fill the emotional void in his life.

His key relationships are central to his development. His bond with Atri, the android girl, is the most significant. He is initially irritated by her incompetence and childlike demeanor, declining her offer to be his "leg." However, her presence forces him to become more sociable and gradually changes his view of the world. A deep, complicated connection forms, partly because he unconsciously recognizes her as the same girl who saved him from attempting suicide when he was nine years old, moments after his accident. He also works with Catherine, the debt-ridden detective and his former teacher, whose plan for the salvage operation sets the entire plot in motion.

Over the course of the narrative, Natsuki undergoes significant development. He transitions from a traumatized, debt-ridden cynic who is haunted by the loss of his mother and leg into a more caring and resolute individual. He learns to accept help and companionship, and he confronts his initial belief that Atri's feelings are merely pre-programmed algorithms, ultimately accepting the genuine nature of their bond. Notably, when Atri's energy is depleted, it is Natsuki who devises the solution to restore her by returning her to her capsule.

In terms of notable abilities, Natsuki is characterized as intelligent and a former student of an elite academy, suggesting a sharp mind. He is resourceful enough to undertake dangerous salvage dives. Despite his physical disability and amputated leg, he manages his daily life with the aid of a walking stick, and his primary personal goal is to overcome this physical limitation by acquiring a more advanced prosthetic limb.