Ikuto Touhouin is the central protagonist who finds himself stranded on an isolated island after a quarrel with his father leads him to run away from home. He boards a cruise liner seeking freedom, only for a violent storm to throw him overboard. After drifting at sea for a week, he washes ashore on Airantou, an uncharted island where he is discovered and resuscitated by a girl named Suzu. The primary motivation that drives him initially is a fierce desire to return to the outside world, a goal fueled by the arguments with his father, who often told him that things were impossible. This history has given Ikuto a powerful, almost berserk, determination; hearing the phrase it is impossible or anything similar triggers a stubborn tenacity that pushes him to accomplish his goals or be knocked unconscious trying.

In terms of personality, Ikuto is generally a gentle, kind, and perceptive young man who lives by his grandfather's teaching to always be kind to girls. However, he is also notably conservative and has a strong sensitivity to situations he considers perverted, such as accidental glimpses of underwear, which frequently cause him to have dramatic nosebleeds and faint. This is complicated by the fact that the island's female inhabitants, having grown up without any men for over a decade, are often unaware of such modesty norms, leading to awkward encounters like mixed bathing. Despite his perceptiveness in other areas, Ikuto is famously oblivious to romantic advances. He genuinely believes that the girls who pursue him do so only because he is the lone human male on the island, rather than because of genuine affection, which leads him to misinterpret their flirtatious gestures as simple friendliness.

His role within the story is multifaceted. He serves as a catalyst for much of the island's social activity as the primary focus of an unwanted harem. Beyond the romantic comedy elements, Ikuto also takes on the practical role of a teacher, educating the islanders in mathematics and science. His deep love for detective novels makes him highly rational and occasionally useful as an amateur detective when mysteries or strange incidents occur on the island. He presents a stark contrast to the island's isolated, traditional Meiji-era lifestyle with his modern, scientifically-minded worldview. A defining character trait is his arbitrary skepticism; he frequently denies the existence of anything supernatural, such as ghosts or yokai, even when confronted with them directly, often explaining them away with bizarre rationalizations like calling a ghost a polar bear. However, he has no trouble believing in concepts like aliens or robot girls, which he considers scientifically plausible.

Key relationships form the core of his experience on the island. He lives under the same roof as Suzu, the girl who saved him, and their innocent cohabitation forms the series' central dynamic. He shares a brotherly mentor relationship with Shinobu, a former shinobi who he trains with and who looks up to him. His presence also influences the entire island community. The four powerful lords who protect the island, including the Northern leader Taiga, become frequent opponents and sparring partners for him. Over time, his appearance is noted to bear a striking resemblance to Takatora, Suzu's missing father, which adds another layer to his integration into the island's history.

Throughout the narrative, Ikuto undergoes significant development, particularly in his abilities. Initially, his physical strength pales in comparison to the superhumanly fit women of Airantou. He realizes his swordsmanship has decayed and begins rigorous training with a mentor named Karaage. Through this training from hell, his skills improve dramatically. He eventually becomes capable of holding his own against the island's four guardian lords, mastering techniques like the substitution jutsu and developing the dexterity to dodge bullets. Despite his prowess with a wooden katana, he is notably bad at ball sports. He also displays a unique and unexplained ability to physically punch ghosts, a consequence of his intense denial of their existence. It is later revealed that his rejection of the supernatural was not merely stubbornness but the result of a spell cast on him by a specialist to protect a family secret about his sister. After this spell is broken, he begins to slowly accept the strange realities of the island. Another distinct trait is his acrophobia, or fear of heights, which he struggles to overcome despite developing impressive ninja-like agility.