Kororo, known as Corey in the English anime, is the spirit ally of Usui Horokeu, also known as Horohoro. Kororo is a Koropokkoru, which translates to "little people under butterbur leaves" in Ainu, and she is surprisingly cute. In the English anime, they are referred to as Minutians. She is Horohoro's spirit ally and has a crush on Manta Oyamada, even though he is human. Her name in the English version sounds similar to "Kouri" (氷), the Japanese word for ice, which is also the element that Horohoro's attacks are based on.
Kororo's true form was a young girl named Damuko. She was Horohoro's childhood friend who had moved from the city. (Source: Wikipedia)
