Mr. Stein

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 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. This grudge increased when most of the students follow Jaden's "sleep" approach; he mentioned that Alexis Rhodes was the last student who listened to his lectures until she finally did the "sleep technique" (mentioned in Episode 114). 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. 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".

In episode 139, Mr. Stein is seen when Jim Crocodile Cook uses the Eye of Orichalcum to look into Jaden's dark mind, this was cut out of the english version of the anime.

Stein is seen one more time prior to Jaden's duel with Yubel, as Jaden remembers all the people Yubel has wronged.

In the Japanese version, Stein is voiced by Mitsuru Miyamoto while Marc Diraison does him in the English version.

Deck
Mr. Stein plays a Scab Scar-Knight Deck, which focuses on the abilities of the eponymous card (based on Unit 01 from Neon Genesis Evangelion) to gain control of his opponents' monsters.