Mr. Stein, known in Japan as Professor Kouji Satou 「佐藤浩二, Satō Kōji」, is a Duel Academy professor only seen in the third season, though flashbacks indicate he was present 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 Yuki for ruining the interest of other students in his lectures because of the boy's position as the academy's top Duelist despite having his horrendous grades. This sets an example for others that they don't need to study to succeed, causing most of them opting to sleep through his classes. His grudge increased when his one good student, Alexis Rhodes, ditched his class, sending him over the edge.
During his days as a pro Duelist, he was also studying to become a teacher so he didn't have much time to practice. He once Dueled The D but fell ill before being able to complete the match due to his extensive studying. It is revealed that when Stein could no longer Duel professionally, "Scar-Knight" "transformed into a bitter hardened version of himself", creating the separate "Scab Scarknight". He tried to prevent Jaden from reaching Thelonious 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 dub, he is characterized as a parody of Ben Stein, particularly Stein's role in Ferris Bueller's Day Off. He is noted as having a dull accent. In the Japanese version, Satou quotes Uncle Ben Parker's famous line "With great power comes great responsibility" from Spider-Man.
Stein is seen one more time prior to Jaden's Duel with Yubel, as Jaden remembers all the people Yubel has wronged.
Stein despises ignorance of any kind, as demonstrated by an analogy he uses: A piece of trash is on the ground, and two people pass it. The first sees it but does not pick it up, while the second does not see it, and while Jaden believes that the first is the worse person for intentionally leaving it there, Stein believes that the second is the worse person because the first might come back to pick it up, but the second never will, since he knows nothing of it. He despises Jaden in particular for having a negative impact towards other students, as his apathy towards actual schoolwork and talent in dueling has inspired other students to stop attending classes, believing that they don't need hard work to become great duelists themselves (while this is a valid complaint against Jaden, Stein's ideology is still rather lopsided since, as noted above, he views outright failing to notice a problem as worse than willfully ignoring it). He also hates Alexis since she stopped attending his class, which he viewed as a form of betrayal given that he had previously considered her an ideal student.
In the English dub, Stein's mannerisms are modeled after actor Ben Stein, as he speaks in a monotone that is comically boring during his lectures (though his lectures are still comically boring in the original version, since not even Bastion is able to sit through them). In both the English and Japanese versions, Stein is polite even as he blames Jaden for ruining everything.