Frost, known in Japan as Koorimaru (氷丸」 Koorimaru) is one of the four Light Brigade members and is called upon by Sarina to be Thunder's Tag Duel partner against Syrus Truesdale and Tyranno Hassleberry. Unlike Thunder, he values victory above all else, and was not above eliminating his partner from the Duel by betraying him and using "Mirror of Duality" to defeat not only Syrus and Hassleberry, but also Thunder. He was then sealed in a mirror by Sarina when she no longer had any use for him, but is freed alongside the other Light Brigade Duelists after Jaden Yuki and Aster Phoenix defeated her.
The "Kouri" portion of his name in the Japanese version is derived from the word "kōri" (氷), meaning "ice." In the English version, he speaks with a Gangsta Rap like Hispanic accent.
Frost plays an Ice Deck, supported by Tag-Team Duel-exclusive cards. His tactics are less upfront than his partner's in that he utilizes cards such as "Mobius Castle" and "Mobius Glacier" to weaken his opponents' monsters, then destroy them through the attacks of "Mobius the Frost Monarch".