Gagin

Gagin commands Kokuboro's Executive Department, with Sakon as his second-in-command. Gagin normally appears in a human skin and is basically a hothead that loves a challenging fight. Most of Kokuboro's motives are secondary to him, he just wants to seek out and defeat strong opponents. He is at first disappointed when he sees that Karasumori's guardians are basically children, but quickly becomes excited when he sees their power. Though he is one of Kokuboro's best fighters, he allows his thirst for battle to overtake his senses.

(Source: Kekkaishi Wikia)