Kūko is a female Cthughan (alien from Cthugha). She takes the form of a slender, red-haired girl. She is in love with Nyaruko despite that her race is the archnemesis of nyarlathotepians. She transfers to Mahiro's school and takes the name Kūko Yasaka, Nyaruko's "wife". At first she is hostile toward Mahiro, but as the story progress, she becomes more friendly and develop feeling for him, and changed her goal to consist of a family with both Mahiro and Nyaruko. She is also obsessed with otaku culture and video games. Kūko was Nyaruko and Hasuta's space elementary school classmate and appears to at the same age as them, although she became a NEET after graduating from high school, and become a Space Defence Agency agent and colleague to Nyaruko later by using the relationship of her uncle who is an employee in the human resource department of the Space Defence Agency. She concealed her feelings for Nyaruko from her family and lied to them that Mahiro is her fiance, as love between Nyarlathotep and Cthugha are considered a shame of the family among Cthughans. She possess the power of fire, able to control fireballs, heat, or drones that fire lasers; she refers to them as space CQC - Cthugha version. Her strength in combat is about the same as Nyaruko.
She is based on Cthugha by August Derleth