Mahiro Yasaka begins as an ordinary high school student until extraterrestrial attacks shatter his normal life, forcing him into constant protection by Nyaruko, an anthropomorphic embodiment of the Crawling Chaos Nyarlathotep. He descends from a lineage of deity hunters, inheriting capabilities like exceptional fork-throwing skills and occasional perceptiveness, though he typically avoids conflict.

He initially resists Nyaruko's chaotic presence and the disruptions caused by Mythos beings, frequently expressing frustration with their behavior and Nyaruko's obsessive romantic advances. This resistance manifests as Tsundere tendencies; he outwardly rejects her declarations of love but occasionally reveals underlying concern, such as desperately seeking a cure during her near-fatal nanomachine infection. His development shows gradual softening, accepting handmade gifts like a knitted scarf and participating in kisses to resolve supernatural problems, though he often maintains an exasperated demeanor.

Key relationship dynamics include being the reluctant object of Nyaruko's Mad Love, facing persistent romantic rivalry from Cthugha (Cuuko), and forming a platonic bond with Hastur (Hasuta). His mother Yoriko displays affectionate, physically expressive care, which he tolerates with mild embarrassment. Later light novel continuity depicts their relationship culminating in a physically overwhelming first sexual encounter, shown through his distressed reactions.

While usually reliant on Nyaruko against cosmic threats, he demonstrates occasional ingenuity, identifying plot-critical items or manipulating alien technology. His psychological resilience surfaces through coping mechanisms like repeating sanity-preserving mantras during chaotic events.

Across all media, Mahiro evolves from a passive victim of cosmic circumstances to someone who acknowledges his complicated bonds with the Mythos beings, balancing his desire for normalcy with reluctant acceptance of their presence.