Natsuki Usami is a seventeen-year-old high school student from the island of Enoshima. He is one of the four main characters in the story. He has short black hair, brown eyes, and typically wears glasses with a perpetually irritated or aloof expression. This outward demeanor stems largely from the death of his mother two years before the events of the series, a loss that left him emotionally closed off and reluctant to form new connections. He is quiet, distant, and can come across as cold or silently judgmental toward strangers, but beneath this exterior he possesses a kind and understanding heart, especially toward people he genuinely cares about.

Natsuki is widely known in the fishing community by the nickname Fishing Prince due to his exceptional skill and his participation in national fishing competitions. He specializes in bass fishing and is also a talented cook. He works part-time at a fishing tackle shop called Hemingway and, during the summer, helps out on a local fishing boat, whose captain he respects as a mentor. Fishing is not just a hobby for him but a deep passion that brings him a rare sense of joy and calm—he shows a bright smile while fishing that contrasts with his usual guarded expression.

His role in the story is that of a skilled guide who teaches fishing to the protagonist Yuki Sanada and the eccentric alien Haru. Initially reluctant and impatient, he gradually warms up to them as they share experiences on the water. His primary motivation is to protect and care for his younger sister Sakura, for whom he shows a soft and gentle side entirely different from his typical demeanor. His relationship with his father is strained; his father runs the family restaurant and wants Natsuki to pursue his own dreams rather than feel obligated to inherit the business, but Natsuki resents his father’s interest in a new relationship after his mother’s death, creating lingering tension.

Over the course of the series, Natsuki undergoes significant development. Through his growing friendship with Yuki and Haru, and his involvement in a larger, mysterious conflict involving aliens and a legendary lure, he learns to open up, trust others, and confront his grief. He also reconciles with his father to some degree, accepting that moving forward does not mean forgetting his mother. His notable abilities include a high level of fishing technique that borders on professional, a strong instinct for reading water and fish behavior, and a practical, no-nonsense approach to problem-solving that proves valuable during the group’s adventures.