Aki Onoda is a twenty-year-old man and one of the three central protagonists of the story. He grew up on a small island where he spent his childhood alongside his two best friends, Ryo Sobue and Yuta Inohara. It was during these early years that Aki discovered a mysterious, hedgehog-like creature in a cave near a Shinto shrine. This creature, which he named Fureru, possessed a magical power that allowed the three boys to hear each other's inner thoughts simply by touching. Aki became Fureru's owner and caretaker, a role he continues to fulfill as an adult.

In his adult life, Aki has moved with his friends to the Takadanobaba district of Tokyo, where the three live together. He works as a part-time bartender and also handles cooking duties, suggesting a practical and perhaps nurturing side to his nature. His personality is defined by a notable struggle with verbal communication. Aki is described as someone who is not a good speaker and tends to act first before speaking his mind. This tendency towards impulsive action, sometimes manifesting as a proclivity for physical violence as an easy shortcut, can be seen as a reflection of his immaturity and his frustration with expressing himself through words. He has been this way since childhood, possessing an inherent awkwardness when it comes to conversation.

Aki's primary motivation is deeply rooted in the bond he shares with his two childhood friends. The trio's connection, facilitated by Fureru's power, is the most important thing in his life. However, the story challenges this seemingly ideal form of communication when a hidden aspect of Fureru's power is revealed. The creature not only allows them to share their true feelings but also has the ability to block out negative thoughts, creating a filtered version of honesty. When this function falters, the friendship is strained, forcing Aki to confront the unvarnished, sometimes painful, realities of his friends' inner worlds. His journey is about learning what it truly means to connect with others beyond a magical shortcut, and understanding that genuine friendship requires accepting imperfection and difficulty.

Within the narrative, Aki serves as the anchor for the group and the primary human link to the creature Fureru. His role becomes crucial during the climax, where he must comfort Fureru when the creature becomes frightened and accidentally traps the trio in a parallel world. He climbs a tower and uses magical threads connected to his finger to reach Fureru, demonstrating his characteristic action-oriented approach to resolving the central conflict. His key relationships are, of course, with Ryo and Yuta, whose dynamic forms the core of the film. He also becomes involved with Nana Asakawa, a young woman the group defends from a stalker, with whom he forms a connection.

Throughout the story, Aki shows notable development. He begins as a character who relies on the telepathic connection with his friends as a crutch to avoid difficult conversations. As the plot forces the group to communicate without this safety net, Aki is compelled to mature. He moves from reacting with frustration and violence to making a conscious, brave effort to reconcile with his friends and understand Fureru's pain. By the end, he accepts that while their connection has changed, the friendship itself can endure, marking his growth from a boy who could only communicate through magic to a young man willing to try with words and actions. His notable ability is not a supernatural power of his own, but rather his unique and long-standing bond with the creature Fureru, which allows him to be the one who ultimately understands and soothes the creature when no one else can.