Nibi is a teenage boy who lives on the Mud Whale, the drifting island at the center of the series. He belongs to a group called the Moles and is best known as the closest friend of Ouni, one of the more prominent residents of the vessel. Nibi has short blond hair that falls to his chin and blue eyes. He typically wears a blue and brown open vest over grey pants and goes barefoot. His personality is boisterous and energetic, often displaying a lively and outgoing demeanor among his peers.

Nibi's motivations are largely tied to his fierce loyalty to his friends, especially Ouni. He is driven by a sense of camaraderie and a desire to protect those he cares about, even in dangerous situations. In the story, he serves as a supporting character whose presence underscores the close‑knit community of the Mud Whale. His most significant role occurs during a critical conflict: Nibi is killed in battle, and his death deeply affects Ouni, triggering the awakening of a powerful, unknown ability within Ouni that allows him to use thymia destructively and eventually destroy an enemy vessel. This event marks a turning point in the narrative, highlighting Nibi's importance not only as a friend but also as a catalyst for Ouni's growth.

Key relationships include his bond with Ouni, which is described as that of best friends. He also interacts with other members of the Moles and the broader Mud Whale society. In terms of development, Nibi does not undergo a lengthy personal arc; his character is defined by his loyalty and his tragic demise, which serves to advance the story and deepen the emotional stakes for the main characters. Notable abilities are not detailed in the available information; he appears to be an ordinary inhabitant of the Mud Whale, without any special thymia power that is specifically highlighted.