"It looks like you've decided how you would survive, Sendou Aichi."
Mark Whiting (Mr. Mark) / マーク·ホワイテイング
Anime Biography: An American with a rich love for Sengoku period history, Mr. Mark is Aichi'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. Unusually, Mr. Mark uses history trading cards in his classroom as a visual aid, and focuses on student participation to help with learning. Although he hasn't been seen since ride 16, Mr. Mark's teaching is essential to Aichi's character development, as it's his encouragement that leads Aichi to start seriously competing at Vanguard.
Spoiler warning: Season 1 details follow.
Outside of school, Mr. Mark operates under the alias of Ninja Master M. Until this year, Ninja Master M had shown up to each of Card Capital's twice-annual shop tournaments to compete against strong fighters with his Nubatama ninja cards. However, he is curiously absent from the second shop tournament in Ride 37, and is instead succeeded by one of his students, Morikawa Katsumi.
Spoilers end here.
Manga Biography: In the manga, Mr. Mark is the sponsor of Hitsue Junior High's CF Vanguard Research club--a club that consists of Morikawa, Izaki and two other students. Because the club members' average on their last test was 27.5, and the passing mark was 40, they were going to be disbanded; that is, until Aichi joined with a mark of 95.
Universal Notes: The reason for Mr. Mark's disappearance is most likely that his voice actor, Abe Atsushi, is also the voice actor of Suzugamori Ren. Since having the two active simultaneously in the series would mean having one actor do double the work, and possibly end up talking to themselves, it's simpler to have Abe only voice one person at a time.
Fan Culture: Next to Emi, Mr. Mark ranks as probably the most beloved secondary character of season 1, despite having nearly zero screen time. His clan has likewise developed a cult following, in no small part due to Bushiroad's complete negligence of it--because of a general perception of the Murakumo "replacing" the Nubatama, their popularity skyrocketed and the Nubatama became a kind of TCG martyr. Most discussion of upcoming booster packs begins with "I wonder if Nubatama will be completed."
Decks and Playstyle
Mr. Mark's deck is a Kagerou-Nubatama mix which gradually reduces the opponent's hand through card skills, to gain a numbers-based advantage in straightforward battle.
Rides 9-11: Nubatama Ninja Forces 16/50 cards
x2 Stealth Beast Hagakure
x1 Embodiment of Spear, Tahr CT
x2 Stealth Dragon, Dread Master
x1 Embodiment of Armor, Bahr
x1 Wyvern Guard, Barri
x1 Flame of Hope, Aermo
x1 Wing Dragon Skyptera
x3 Stealth Beast Chigasumi
x1 Dragon Knight Nehalem
x1 Exile Dragon
x2 Stealth Dragon, Void Master