Kūko (クー子, also known as Kuuko and Cthuko) is one of the main characters, and an alien who is in love with Mahiro and she appears to be clingy. A female Cthughan (alien from Cthugha). She is hostile toward Nyaruko and tells her to stay away from Mahiro in the beginning of the story. As the story progress, she becomes more friendly towards her. She transfers to Mahiro's school and takes the name Kūko Yasaka, Mahiro's "wife". 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, She possesses the power of wind, able to control blizzards, gusts, or winds. Her strength in combat is lower than that of Nyaruko's.
While her original form as an alien remains to be seen, her from throughout the series is always a teenage girl with two long ponytails that fall down to her waist in length. Her eyes are red, and her skin is somewhat pale, although whether it is a side effect of her gaming or intended is unknown.
When she is at school, she is constantly wearing the same uniform as the rest of the female students. That is a light blue long-sleeved coat with white accents. She also wears a pink bow tie and black knee-length socks.
Kuuko has a very interesting persona, as her love for video games are quite unlike one would expect of an extraterrestrial. She is a kind person at heart, but can be extremely perverted when things get out of hand.
They were once on somewhat hostile terms, what with Nyaruko loving Mahiro over Kuuko. This led to the initial attempt and desire to fight over him. However, after Kuuko began residing in the household, the two have become good friends. Despite their history, Nyaruko finds her a reliable person. She has come to respect her as a friend.
To say that Kuuko is in love with Mahiro is not an exaggeration, although her sexual deviancy tends to leak out and have her say highly inappropriate things sometimes. What exactly made her fall for Mahiro is not known, but she seems to love him enough to spend a lifetime and more with him. Unfortunately for him, Mahiro does not share any compassion or reciprocation towards her feelings. This leads to constant comic relief among the group.