Man-At-Arms is the primary title of Duncan , a fictional character in the Masters of the Universe franchise. The figure's tag-line is "Heroic Master of Weapons".
In Filmation's animated series, Man-At-Arms - given the "real name" of Duncan - is developed into not only being a master of weapons, but a skilled inventor, soldier and Teela's foster father (something already mentioned in a couple of pre-Filmation stories), as well as mentor to Prince Adam. The Sorceress of Castle Grayskull asked Man-At-Arms to adopt Teela because she felt that she could not raise her daughter and protect Castle Grayskull at the same time, and believing that if other's knew of Teela's true destiny, it would make her daughter a target. [http://www.dvdtalk.com/reviews/16850/best-of-he-man-and-the-masters-of-the-universe-the/]
Man-At-Arms wears armor which is slightly different from that of the standard uniform of the royal guards. His moustache was not present on the original figure, having been added by Filmation to make him look older, wiser and more fatherly - although original prototype sketches for the Man-At-Arms figure gave him a goatee beard. He is often pestered by court jester Orko, as Orko's magic tricks often backfire directly at him. Duncan is also one of the three people who know He-Man's secret identity, as explained in the introductory sequence of the original cartoon.
Along with Adam / He-Man and Teela, Duncan is one of the cartoon series' most central and most often seen characters.
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Official Site: Mattel
Links: Wikipedia - Duncan He-Man Wikia - Duncan ComicVine - Duncan