The secret identity of He-Man is a primary plot point in most Masters of the Universe fiction. The secret that Prince Adam and He-Man are the same person (and Adam's pet Cringer is He-Man's mount Battle Cat) creates conflict and dramatic tension in many stories. As explained in the opening of He-Man and the Masters of the Universe, only The Sorceress, Man-At-Arms, and Orko know Adam and Cringer are He-Man and Battle Cat. However, the series and its spinoffs would later include other characters, most notably Zodac and She-Ra.
As Adam, He-Man lacks the power and freedom from his royal responsibilities to effectively serve as Eternia's champion, and he is forbidden to transform when it might expose his secret. Thus, He-Man's greatest liability is the possibility that he will be trapped as Prince Adam during a crisis. Additionally, Adam must act irresponsible and careless to dissuade anyone from suspecting his dual identity, causing his friends and family to express disappointment in him.
He-Man had no alter ego until the Paul Kupperberg stories published by DC Comics. "From Eternia -- With Death!" contrasts this dual identity with that of Superman--whereas Clark Kent hides his secret by acting meek and powerless, Prince Adam acts reckless and immature. Although Adam freely flaunts his great strength before Man-At-Arms and Teela, they regard him as too headstrong and undisciplined to ever confuse him with the stoic, responsible He-Man.
To transform into He-Man and Battle Cat, Adam and Cringer venture to the Cavern of Power, where they are instantly changed into their superheroic garb. It is not made clear in the Kupperberg stories whether He-Man and Battle Cat have any abilities that Adam and Cringer lack; the pair may simply hold back in their normal forms.
Masters of the Universe #2 reveals that Adam first beccame He-Man after discovering Ceril's people being menaced by the evil sorceror Damon. While Adam battled Damon, the Goddess sent Zoar to magically transform him into He-Man. As these events predate Adam's association with Cringer, the origins of Battle Cat are not explained.
The character of Prince Adam was refined in the original He-Man and the Masters of the Universe animated series. In stark contrast with the sword-and-sorcery style of the early comics, He-Man is a paragon of the moral virtues intended to be taught to the show's audience; accordingly, Adam is just as noble, polite, patient, and wise as his alter ego. Instead of wenching and carousing, Adam disguises his secret more innocently, by acting lazy or inept. This creates tension between him and King Randor, who expects Adam to prepare to be king, and Teela, who is obligated to train and protect Adam.
It is implicit throughout the series that none of the characters notice that Adam and He-Man look exactly alike (save for their clothing and skin tone), wield the same sword, and both associate with talking green tigers. Even Man-At-Arms, knowing Adam is He-Man, could not recognize Battle Cat as Cringer when they first met in "Battlecat." This conceit is particularly strained in episodes such as "Valley of Power" and "The Sleepers Awaken," in which Adam and Teela are the only humans for miles around and yet Teela finds it plausible that He-Man would wander by just as Adam disappears. The possibility that He-Man would even have an alter ego is not even considered by most characters, except in "Mistaken Identity."
Although the series opening explicitly identifies the Sorceress, Man-At-Arms, and Orko as the only people who know Adam and Cringer are He-Man and Battle Cat, Zodac is shown to be aware of the secret in "The Search." In "The Rainbow Warrior," Adam becomes concerned that Queen Marlena has figured out the secret, although Marlena avoids letting him confirm his suspicions. Princess Janice is established to be an unusually bright young girl in "A Trip to Morainia," and seems to prove this by commending Adam for He-Man's actions, although nothing comes of this development. It is probable that omniscient beings such as Om and the Oracle of the Crystal Sea either know He-Man's secret or have the means to discern it, although this is never stated onscreen and such characters would probably have little use for the information.
At times Adam appears to be careless about revealing his dual identity. In both "Teela's Trial and "Journey to Stone City," the Attak Trak is either aware of the secret or completely oblivious to the fact that it transported Adam miles from civilization and then left him behind after picking up He-Man. In "Quest for He-Man," Ram Man does not seem to notice that although he brought Cringer with him to look for He-Man in another dimension, it is Battle Cat that returns home with his party. Mallek is present when He-Man transforms Cringer into Battle Cat in "The Witch and the Warrior." None of these incidents are treated as serious plot points, and it is probable that the writers and directors of these episodes simply overlooked the inconsistency.
By extension, those who know She-Ra's secret identity also know He-Man's, and vice-versa.
The minicomics accompanying the first wave of He-Man show Skeletor tricking the Galactic Guardians Hydron and Flipshot to transfer the power of Grayskull to Starship Eternia, so Prince Adam must aboard the Starship and transform in front of Skeletor to recover the power, so the secret is found out. Hydron and Flipshot are also present, but it's unknown if they find out the secret as they never hear Adam's name save for being referred as "Prince has-been" by Skeletor. The point gets moot, however, when the Sorceress announces that from now on, He-Man will only be He-Man, without any other identity.
This development is also followed by the German comics published by Ehapa, where He-Man remains as He-Man the whole time he is in Primus.
The New Adventures of He-Man, despite being based on the He-Man toyline, keeps the secret identity of Adam when he comes to the Tri-Solar System, as the writers thought the transformation sequence was too iconic to lose it.
In Primus, Adam reveals his identity to Master Sebrian, who presents him as his nephew from the city of Levitan. Mara suspects that Adam is He-Man, mainly because of Adam's ignorance of some of the customs at Primus, but she never outright finds it out.