Coo-ko (also known as Cthugha or Kuuko) is an alien entity from the Cthugha species, often called "The Living Flame." She is Nyaruko's childhood friend, having known her since kindergarten and elementary school. Despite the historical enmity between their species, the Nyarlathotepians and the Cthugha, Coo-ko harbors deep romantic feelings for Nyaruko. This persistent affection is frequently met with rejection, as Nyaruko prioritizes her relationship with the human Mahiro Yasaka.

Typically reserved and shy, Coo-ko often blushes in social situations. She presents as quiet and introverted, contrasting sharply with Nyaruko's exuberant personality. Nyaruko mockingly labels her a "NEET" due to her intense passion for video games and otaku culture, aligning with her reclusive tendencies. Visually, she possesses distinctive red hair that sometimes shifts shape in response to strong emotions, such as forming a heart during romantic thoughts.

Within narrative arcs, Coo-ko participates in key confrontations, including the assault on the R'lyeh theme park headquarters where she joins Nyaruko and allies against adversaries like Nodens. Her abilities include pyrokinesis, consistent with her species' fiery nature. Her unrequited love for Nyaruko remains a recurring element throughout the series, contributing to interpersonal tensions. Her background as Nyaruko's former classmate and her ongoing efforts to navigate their complex relationship form a consistent aspect of her character development.