Mr. Mark is Aichi Sendou's history teacher at Hitsue Middle School, and opens the series with his narration on the Second Battle of Kawanakajima(1555) between Takeda Shingen and Uesugi Kenshin. Mr. Mark uses history trading cards in his classroom as a visual aid, and focuses on student participation to help with learning. Mr. Mark's teaching is essential to Aichi's character development, as it's his encouragement that leads Aichi to start seriously competing.

Outside of school, he operates under the alias of Ninja Master M.

In the third season, Mark becomes a teacher at Hitsue High School and is also the faculty advisor for the school's Vanguard Club.