Nyarlathotep, known as Nyaruko, originates as an extraterrestrial Outer God from the Cthulhu Mythos, specifically the entity called the Crawling Chaos. In this adaptation, she takes on a humanoid form resembling a silver-haired teenage girl with blue-green eyes, usually dressed in a school uniform. She is dispatched to Earth by the Space Defense Agency with the mission to protect high school student Mahiro Yasaka from intergalactic threats, primarily alien traffickers and smugglers targeting Earth's entertainment media like anime and games. This assignment leads her to reside with Mahiro as his self-appointed guardian.

Nyaruko exhibits a hyperactive, impulsive, and overtly affectionate personality. She frequently expresses obsessive romantic interest in Mahiro through aggressive advances, including tackling him or invading his personal space. Despite her chaotic demeanor, she demonstrates unwavering loyalty and strong protective instincts. She often employs her cosmic abilities in exaggerated ways, transforming everyday objects like forks or pans into weapons against adversaries during comedic combat. Her behavior blends childlike enthusiasm with moments of genuine concern for Mahiro's safety, though her actions typically escalate into slapstick violence.

Her background as an Outer God grants her vast supernatural powers, including shape-shifting, energy projection, and rapid regeneration. She references her ability to assume other forms, though her primary appearance remains humanoid. She commands a pet Shantak from Lovecraftian lore, which occasionally causes minor household disruptions. Her combat style frequently parodies tokusatsu genres, complete with shouting attack names inspired by Japanese superhero media.

Nyaruko forms connections with other Mythos-based entities living on Earth. Her closest companions include Cthugha (Kuuko), a fiery-tempered entity depicted as a red-haired girl, and Hastur (Hasuta), a male entity who often cross-dresses and harbors feelings for Mahiro. Later, she interacts with Atoko Shirogane, an avatar of the Great Old One Atlach-Nacha, who assists her on missions while displaying eccentric behavior. Nyaruko also introduces Mahiro to Nyarue, an original Outer God friend prone to transforming into her true eldritch form when flustered.

Character development occurs through her evolving relationship with Mahiro. Initially dismissive of her affections, he slowly warms to her presence and acknowledges her dedication, particularly after instances where she sacrifices personal desires for his well-being. Key moments include a body-swap incident forcing her to experience human vulnerability firsthand and a mission requiring her to temporarily leave Earth to protect it, during which she extracts a promise from Mahiro to reunite. Her emotional depth surfaces in quieter scenes, revealing insecurities beneath her exuberant facade.

Her role expands beyond comedy through missions for the Space Defense Agency, such as retrieving limited-edition figures to create planetary defense barriers or confronting entities like Ghatanothoa. These plots highlight her adaptability, blending cosmic responsibilities with mundane human activities like part-time jobs or hot springs visits. Throughout, her foundational traits as Nyarlathotep—messenger of the Outer Gods and embodiment of chaos—remain consistent, reinterpreted through a lighthearted lens.