Keiji Hase is a central character who serves as the best friend and classmate of the protagonist, Yushi Inaba. Coming from a wealthy and influential family, Hase is initially presented as someone who seems almost unnaturally perfect. He is good-looking, cool-headed, exceptionally skilled at nearly everything he attempts, and possesses significant financial resources. This surface-level perfection, however, is just one facet of a more complex individual, and his role in the story is defined less by his own supernatural journey than by his unwavering support for Yushi and his unique place within the extraordinary community of Kotobuki-so.

Hase's personality is a compelling blend of contradictions. In ordinary social settings, his charm, intelligence, and cunning can make him appear somewhat devious, a trait that is often played for comic relief. Yet, when he visits the yokai apartment, his demeanor shifts dramatically. Surrounded by the deeply quirky and supernatural residents, the ostensibly perfect Hase becomes the most ordinary person in the room. This contrast humanizes him, revealing that his true strangeness is not his coolness, but his complete and enthusiastic acceptance of the apartment's bizarre world. His childlike adoration for the ghost child Kuri is a prime example of this, as he dotes on the young spirit with a sincerity that borders on the geeky, willingly earning the title of Kuri's Daddy from the other residents. This deep affection for Kuri is a key motivation for his frequent visits.

Within the narrative, Hase's primary motivation is his friendship with Yushi. He is a grounding force for the protagonist, acting as a friend who sees Yushi clearly and is unafraid to speak honestly, even when the truth is difficult to hear. This dynamic is crucial for Yushi's personal development, as Hase provides an external, objective perspective that helps Yushi understand himself and his growth. Beyond his emotional support, Hase often uses his wealth in practical, generous ways. He arrives from his trips loaded with personalized gifts for everyone in the apartment, from a thoughtful ring for Ruriko to numerous plush toys for Kuri. On one notable visit, he even pays the entire month's rent and provides a food allowance, making him a beloved, almost legendary, guest.

His key relationships revolve around the residents of Kotobuki-so. He shares a strong, long-term friendship with Yushi, which is a cornerstone of the series. He is exceptionally fond of the ghost child Kuri, to the point where the running gag of him being the father figure, with Yushi as the reluctant mother, highlights their tight-knit, found-family dynamic. He also gets along well with the other quirky residents, such as the eccentric Antiquary, finding a willing participant in the old man's odd hobbies. His genuine respect for the adults in the apartment, who treat him as a normal person rather than sucking up to his family's status, further motivates his attachment to the place.

Throughout the story, Hase's development is less about acquiring new powers and more about revealing the depths of his character. His visits often underline Yushi's progress, acting as a barometer for how much the protagonist has changed. As for notable abilities, Hase does not possess any supernatural skills. His strengths are entirely worldly: immense wealth, high social intelligence, a sharp and cunning mind, and an almost supernatural talent for giving the perfect gift. His true power lies in his loyalty and his function as a bridge between Yushi's old, ordinary life and his new, magical one, offering a sense of normalcy and unwavering support within the chaos of the yokai world.