Omi Kiriyama

Omi is a second year student at Kouta's school who is in fact a sickle weasel with the power to control the wind. Omi was once the bitter enemy of the Minamoto siblings. In the fourth volume of the novels, he succeeds as the gang leader of the delinquents at his school. He hates the friendship between Kouta and his friends, and does not speak much. In the anime, he accidentally hurts Kouta who took a blow for Chizuru.
Source: Wikipedia.