Kororo (Corey in the english anime) is the spirit ally of Usui Horokeu, nicknamed Horohoro.

Kororo is a Koropokkoru (which means "little people under butterbur leaves" in Ainu, and is surprisingly cute. (Known as Minutians in the English anime), and Horohoro's spirit ally. She has a crush on Manta Oyamada, even though he is human. Her name in the English anime version sounds similar to "Kouri" (氷), which is the Japanese word for ice and also the element that Horohoro's attacks are based on.

(Source: Wikipedia)