Mahiro Yasaka is a Japanese high school student who sports dark brown hair and grey eyes. His ordinary life is upended when extraterrestrial entities target him, leading to an encounter with Nyaruko, a cosmic deity who protects him. This event results in Nyaruko moving into his home and enrolling in his school, joined by fellow cosmic beings Kūko and Hasuta, who also adopt the Yasaka surname.

Initially presented as an unremarkable student, Mahiro possesses a serious and grounded personality. He consistently serves as the rational counterpoint to the otherworldly chaos brought by the cosmic entities surrounding him. His primary defense against Nyaruko's persistent romantic advances involves stabbing her with a fork, a distinctive defensive technique inherited from his mother, Yoriko Yasaka. Yoriko's part-time work combating supernatural threats, utilizing similar fork-based methods, underscores this unique familial ability to harm cosmic entities and shapes Mahiro's background.

Despite his outward resistance to Nyaruko, Mahiro develops significantly emotionally. He harbors underlying feelings for her but struggles to reconcile these emotions with her true, chaotic cosmic nature. A pivotal moment occurs when Nyaruko, facing memory loss, declares her enduring affection, prompting Mahiro to acknowledge his own feelings. This leads to an emotional exchange where they kiss. Although both subsequently lose memories of this incident, their relationship progresses, culminating in a public kiss at school.

As the series progresses, Mahiro gradually adjusts to sharing his life with multiple deities, though he maintains his readiness to use the fork when necessary. His interactions within this extraordinary social circle gain further complexity through relationships like Kūko's rivalry and Hasuta's affection.