Mark Whiting is a history teacher at Hitsue Middle School and later at Hitsue High School. An American with a deep enthusiasm for Sengoku period history, he opens the series with a narration on the Second Battle of Kawanakajima between Takeda Shingen and Uesugi Kenshin. In the classroom, he uses history-themed trading cards as a visual aid and places strong emphasis on student participation, making his lessons engaging and memorable.

His personality is both earnest and eccentric. As a teacher, he is encouraging and invested in his students' growth. Outside of school, he adopts the alias Ninja Master M, an eccentric cardfighter who dresses and acts like a ninja, revealing a playful side. His motivation stems from a genuine desire to inspire learning and confidence in others, particularly through the game of Vanguard.

Within the story, Mark Whiting serves as a mentor figure. His encouragement is a key factor in Aichi Sendou's decision to begin competing seriously in Vanguard tournaments. By the time of Asia Circuit Hen, he has transitioned to teaching at Hitsue High School and becomes the faculty advisor for the school's Vanguard Club, supporting the main cast from an educational and organizational standpoint.

His most significant relationship is with Aichi, whom he teaches both history and the value of persistence. He also interacts with other members of the school community as a trusted authority figure. Over the course of the series, Mark develops from a minor supporting character into a more prominent role, eventually even becoming a playable figure in the Shinemon arc. His notable abilities include his skill as a cardfighter under his ninja persona and his innovative teaching methods that blend history with card games.