Ku-ko is a Cthughan alien from the Cthulhu Mythos reimagined as extraterrestrial. She first met Nyaruko as childhood classmates in space elementary school, forging a dynamic where Ku-ko views Nyaruko as both a rival and the focus of obsessive romantic affection. This duality defines their interactions. Assigned to Earth, she joins Nyaruko and Hasuta in guarding Mahiro Yasaka against hostile aliens. Ku-ko typically maintains a stoic, reserved exterior, contrasting with Nyaruko’s overt enthusiasm. Yet this masks fierce possessiveness toward Nyaruko, triggering aggressive, comedic attempts at physical intimacy despite Nyaruko’s protests. She enrolls at Mahiro’s school as "Kko Yasaka," adopting the surname to reinforce her self-declared bond with Nyaruko, whom she insists is her "wife."

Beyond pursuing Nyaruko, Ku-ko displays deep fascination with Earth’s otaku culture, especially video games, mirroring Nyaruko’s obsession with human media. In combat, she wields fire-based powers intrinsic to her Cthughan nature.

Her character evolves notably during the second season. When her cousin Kune arrives to enforce a childhood marriage pact, Ku-ko participates in a ruse where Mahiro poses as her lover to deter Kune. This forces introspection about her feelings. While her primary devotion remains fixed on Nyaruko, the deception with Mahiro hints at subtle, unexpected shifts in her perception of him beyond mere rivalry.

Despite these nuances, Ku-ko’s core traits endure. She retains her monotone delivery yet erupts into passionate outbursts directed at Nyaruko. Her role persists as both a protector of Earth and Mahiro and a source of chaos through her single-minded romantic pursuits and comedic clashes rooted in her rivalry with Nyaruko.