Duncan, the personal name established in S2E4 ‘The Problem with Power,’ is one of children’s animation’s most fully-realized mentor figures. He is the one Heroic Warrior who knows Adam is He-Man. He is the adoptive father to Teela. He is the chief weapons inventor of Eternia. He is the king’s strategic advisor. He carries all of these roles without ever quite becoming the show’s protagonist. The character’s structural function is to be the trusted-and-trusting figure in Adam’s life that Adam’s birth father (King Randor) cannot quite be. Alan Oppenheimer voiced both Man-At-Arms and Skeletor in the Filmation series — the show’s mentor and the show’s villain delivered by the same actor.
Catalogue source: heman.fandom.com (CC BY-SA 3.0).