Man-At-Arms is the primary title of Duncan, a character in the Masters of the Universe franchise. The role of a Man-at-Arms in the government of Eternia was to advise the ruling planetary monarch and the Royal Guard on military and police affairs.
In the early minicomics Man-At-Arms is portrayed somewhat differently. His appearance is more in line with the toy, and as there was no royal family yet there is no mention of them. In the first minicomic there is also no mention of his relationship to Teela, who is instead portrayed as a 'warrior goddess'.
Instead, the only background given for the character in He-Man and the Power Sword is that his people were "Masters of all weapons." He seems to be well known in this version of Eternia, but little information beyond this is given.
For the second wave of minicomics, Man-At-Arms is first referred to as Teela's father. the minicomic The Tale of Teela! reveals that years ago he saved the Goddess from having all her power drained by a magical clone created by Skeletor, and since the Goddess' duties impeded her from raising the now powerless clone, Man-At-Arms raised her as his own daughter and named her Teela.
- Main article: Man-At-Arms (early minicomics)
In the stories written by Paul Kupperberg for DC, Man-At-Arms is unaware that Prince Adam and He-Man are one and the same. Because of this, he displays as much disdain for Adam's apparent lack of interest in being anything more than a womanizing layabout as Teela does.
- Main article: Man-At-Arms (DC Comics)
In the 1980s animated series, Duncan is an inventor, soldier and Teela's foster father, as well as a mentor to Prince Adam. The Sorceress of Castle Grayskull asked Man-At-Arms to adopt Teela because she felt she could not raise her daughter and protect Castle Grayskull at the same time, begging: "Please, take care of my daughter." Man-At-Arms complied: "She will always be safe with me for as long as I live."
Man-At-Arms wears armor which is slightly different from that of the standard uniform of the royal guards. His mustache was not present on the original figure (which was also beefier and more menacing), having been added by Filmation to make him look older, wiser and more fatherly. Man-At-Arms has an understanding, fatherlike voice. He is often pestered by court jester Orko, the 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 cartoon. Also, Man-At-Arms knows that Adam's twin sister Adora is She-Ra. 
More than an ally and friend to Adam, he sometimes shows him a great fatherly regard as a second father, especially when Adam has to leave the scene, taking his defense by telling King Randor that Adam has an understanding of his duties as a prince.
The 2002 cartoon portrays Duncan as a much more serious and darker character, with more of a connection to the Sorceress than was previously shown. As in the Filmation series, Man-At-Arms is Teela's adoptive father, but he is also depicted as an officer with the definite rank of General instead of the undefined, but still high-echelon position he holds in the Filmation cartoon. Likewise, Man-At-Arms has a close mentor/student relationship with Adam in addition to aiding He-Man and leading the Masters (as the Heroic Warriors are called in the 2002 version). It is also revealed that he is the brother of Fisto. It also revealed Duncan's mentor, Dekker. In this series, Man-At-Arms was voiced by Gary Chalk,
Masters of the Universe Classics toy line
Although the Masters of the Universe Classics line has, as yet, no supporting fiction, the toys' packaging do include short character biographies that merge elements from various different incarnations of the franchise as well as some newly-developed information. His biography on the back of his card for his action figure reads as follows:
- For two centuries, the Eternian weapons master and combat instructor to the royal family has been called the “Man-At-Arms®.” Trained by the renowned tactician Dekker™ and a veteran of the Great Unrest, Duncan™ was asked by King Randor™ to step into this position and fortify his guard with an elite strike force, which he named The Masters of the Universe®. In addition to his skills in combat, Duncan™ is also a great inventor and helped construct an electronic version of the Power Sword® for Adam to use until he could unite both halves of the sword of King Grayskull™. Man-At-Arms® and his adopted daughter Teela® often scout the borders of Eternia® for signs of lurking evil.
- He-Man and the Power Sword
- Battle in the Clouds
- The Vengeance of Skeletor
- The Ordeal of Man-E-Faces!
- The Menace of Trap Jaw!
- The Tale of Teela!
- The Magic Stealer!
- The Power of ... Point Dread!
- Dragon's Gift
- Double-Edged Sword
- Temple of Darkness!
- Siege of Avion
- The Story of She-Ra
- Skeletor's Dragon
- The Battle of Roboto
- Grizzlor: The Legend Comes Alive!
- Hordak: The Ruthless Leader's Revenge!
- The Stench of Evil!
- Mantenna and the Menace of the Evil Horde!
- The Treachery of Modulok!
- Leech: The Master of Power Suction Unleashed!
- The Flying Fists of Power!
- Between a Rock and a Hard Place!
- The Menace of Multi-Bot!
- Energy Zoids