Mr. Stein known in Japan as Professor Kouji Satou, is a Duel Academy professor only seen in the third season, though flashbacks indicate he was there in the first year seeing as Chumley appears in one of them. A former Pro League duelist who came from a poverty-stricken family, Mr. Stein fosters a grudge towards Jaden for ruining the interest of other students in his lectures due to the boy's lazy approach to lessons. He once dueled The D but fell ill before being able to complete the match. (This was because he was forced to study to become a teacher. He was so exhausted he could not continue. According to his flashback when Stein could no longer duel professionally "Scar-Knight" "transformed into a bitter hardened version of himself" creating the separate Scab Scar-Knight.) He prevents Jaden from reaching "Professor Viper" in the lab where the monkey Wheeler was trained, but is defeated. Afterwards, when he attempts to pick up his precious "Scar-Knight" card, he seemingly falls to his death into the abyss. In the English version, he is characterized as a parody of Ben Stein, particularly Stein's role in Ferris Bueller's Day Off.