Kuuko, alternately called Cthuko or Kūko, is a flame-wielding member of the ancient Cthugha alien species, capable of manipulating fire and operating orbital combat turrets. Her human guise manifests as a pale-skinned teenager with crimson eyes and waist-length twintails, frequently adorned in a school uniform comprising a light blue jacket, pink bow tie, and black knee-high socks.

A childhood companion of Nyaruko and Hasuta from their extraterrestrial schooling years, Kuuko nurtures unrequited romantic feelings for Nyaruko. Initially antagonistic toward Mahiro Yasaka due to Nyaruko’s affection for him, she adopts the alias Kūko Yasaka to enroll in his school and reside in his home, gradually forging a bond of mutual reliance with him.

Her demeanor merges stoic reserve with mischievously suggestive remarks, often directed at Nyaruko. A self-identified NEET, she obsessively consumes otaku media and dominates gaming marathons. Combat-wise, she rivals Nyaruko’s prowess, employing incendiary abilities and deployable mechanized turrets as her primary armaments.

Kuuko contends with her clingy older sister Kuune, whose persistent affections parallel Kuuko’s own pursuit of Nyaruko. To deflect Kuune’s attention, Kuuko stages a fake engagement to Mahiro, briefly straining their relationship. Later storylines, including the OVA "Haiyore! Nyaruko-san F," depict her reevaluating her approach to love while retaining her role as both comedic foil and protector within the group.

Her narrative arc traces a journey from obsessive fixation on Nyaruko toward tempered emotional growth, yet her defining attributes—unyielding loyalty, combat expertise, and fervent otaku passions—persist throughout her evolving relationships.