Master Shimo, also known as Mister Down, mentors Harubaru Hinode and Raichi Kuronashi in Bakugan brawling from the Bakugan Dojo. His name, derived from the Japanese word "Shimo" (下) meaning "under" or "lower," stands in direct contrast to his rival Master Jyou, whose name signifies "upper."

In his youth, he trained as Mister Down alongside Master Jyou under their teacher, Quilt. Although Jyou was the superior brawler, Quilt entrusted the Dojo to Shimo due to Jyou's inflated ego. This decision caused Jyou to depart, vowing lasting revenge. During this period, Shimo specialized as a Pyrus-attribute brawler, partnered with the Guardian Bakugan Blue Blaze Gus Burnan.

Beyond his mentorship, Shimo frequently serves as an official referee. He presided over Harubaru and Raichi's inaugural duel at the Bakugan Dojo, awarding Harubaru metal cross parts as a prize. He also appears as a referee during official BAC tournament events across Japan, noted as a namesake appearance.