Calvin is a human boy living in the human world, who serves as a brief but pivotal figure in Onari's journey. He is specifically identified as a gaijin, a Japanese term for an outsider or foreigner, having moved to Japan from the United States of America with his mother. His background as an immigrant shapes his entire worldview and becomes the foundation for his connection to the story's central themes.

In terms of personality, Calvin is portrayed as observant, kind, and empathetic. When he encounters Onari after she has been cast out from Kamigami Village, he does not pry or offer empty platitudes. Instead, he offers her food, specifically a peanut butter and jelly sandwich, an act that provides simple comfort during her distress. He is also reflective and wise for his age, capable of articulating complex feelings of alienation. He demonstrates patience by acknowledging Onari’s pain and choosing to sit with her and share his own story rather than trying to immediately solve her problems. His deep interest in Japanese myths, legends, and fairytales reveals a curious and determined nature, as he used this study as a way to find belonging in a culture that initially treated him as an outsider.

Calvin’s primary motivation stems from his own experiences with discrimination and loneliness. His key action in the story is to comfort Onari after the villagers reject her for being an Oni, which is a human. He recognizes the same pain of being feared and misunderstood that he felt when he first arrived in Japan, where people looked at him as if he were a monster. His goal is not to convince Onari to return to the village that hurt her but to share his own journey of acceptance. He explains that while he is still a gaijin, he has come to love the country and accept it as his home. This story encourages Onari to accept who she truly is and to confront the shadow of fear threatening her village.

Calvin's role in the story is that of a mirror and a mentor, despite his brief appearance. He acts as a catalyst for Onari’s character development. At her lowest point, when she has decided to abandon her home forever, Calvin provides the crucial perspective she needs to reframe her identity not as a shameful secret but as an integral part of herself. He is the one who articulates a major theme of the narrative: that fear and discrimination arise from a lack of understanding. His insight that historians suggest the concept of Oni originated from people who simply look different from the local populace directly challenges the prejudice Onari faces. His friendship also creates a meaningful parallel between the spirit world and the human world, showing that the struggle against fear and xenophobia exists on both sides of the bridge.

Regarding his development, Calvin has already completed his primary character arc before he meets Onari. He is introduced having already overcome his initial feelings of hatred and alienation. He has moved from being a fearful outsider to someone who has actively learned about and embraced Japanese culture, thereby transforming his pain into knowledge and acceptance. His role is to impart the wisdom gained from this journey to Onari, serving as an example that it is possible to find a home and a sense of self even when others perceive one as a monster.

Calvin does not possess any notable supernatural or mystical abilities. He is depicted as an ordinary human child with no magical powers or combat skills. His notable abilities are entirely emotional and intellectual: he has a high degree of emotional intelligence, the ability to listen and comfort someone in profound pain, and the wisdom to articulate complex societal issues like discrimination and belonging in a way that is both simple and powerful. His strength lies in his empathy and his personal story of resilience.