The case where a being is able to engage in superheroics or grand villainy not by gaining any powers or by being a Badass Normal, but by being a vessel in some fashion for another personality that does have superpowers, but only when that personality takes control.
This personality can also look and be physically very different. This takes several superhero/villain issues and solves them in a single stroke. They don't need a disguise because you wouldn't be able to tell by looking at them. Normally, however, writers will not remove the issue of being discovered; they'll use the single moment of transformation as a time where the secret can be discovered or requires the hero diving off into that phone booth. They just don't have to constantly worry about the issue. They can keep the superhero relatable by having the non-powered character be The Everyman or Ordinary High-School Student. They also won't have the same "using your powers for selfish gain" issue because a complete personality change can be used so easily.
The transformation can often be handled by a Transformation Sequence. Issues about how the personality is split off can vary. Sometimes it's like it's just another person hiding inside their body until the right moment, other times it's a total personality shift but they'll retain the memories of being that Superpowered Alter Ego and it will still be them. Maybe some Fan Wank, or even deliberate ambiguity, will occur over how much the alter ego is a separate personality at all or whether it's more of a Super Mode or Henshin Hero with a confidence boost.
- D.N.Angel is based around the main character, Niwa Daisuke, having a long family tradition of being possessed during their adolescence by 'Kaitou Dark,' a supernatural famous art thief. He's tall and handsome where Daisuke is short and cute, and does not act a few hundred years old by any means. But killing the stuff they steal is saving the world or something, so it's reasonably heroic. Mostly his real super powers don't get used much, because it takes a lot out of Daisuke when they are.
- Hilarity Ensues whenever accidental transformations happen (it's set off by his 'heart pounding' in connection with the girl he likes), especially at school. Occasionally it will be Daisuke in charge of Dark's body, but when things get really sticky is when Dark is running things but looks like Daisuke.
- Daisuke's mother was heartbroken that she, as a girl, wasn't able to be Dark, so she fixed on having a son who was and runs his 'career' capably. Tennis Mom of stunt robbery.
- Sort of used in Cardcaptor Sakura with Yukito and Yue; Yue's a magic-using Winged Humanoid, while Yukito is his "false form" that (presumably) Clow created for Yue to hide in while waiting for the Final Judgment. Besides that, Yukito is a normal teenage boy who is unaware of his superpowered true form.
- Birdy the Mighty: Tsutomu can't use the powers of Birdy, even though it's technically her body, but they can communicate and switch with a moment's notice (without invoking a Henshin Hero) once they've agreed they need to work together. Sometimes though, Birdy just steals him away when she needs to run off to do her work.
- The eponymous character(s) of Yu-Gi-Oh! is/are this; upon solving the Millennium Puzzle, Yugi Moto becomes host to the soul of an ancient unnamed pharaoh, who apparently is even better than Yugi is at children's card games. The pharaoh also looks almost exactly like Yugi, but is slightly taller and more mature. While not super-powered per se, the 3-5000 year old pharaoh has an almost supernaturally good grasp of the strategy of gaming, and confidence to match. Oh, and he can attack people's minds with his forehead eye.
- Is "mind crush" your answer to everything?
- Played with, at least in the original manga. One of the themes of the story is that Yugi is stronger than he thinks, being able to surpass his counterpart in some ways (for example, in terms of morality); in the end, it's demostrated through the Ceremonial Battle he's able to surpass Yami in every way, including games.
- Kyo Kara Maoh! has Yuuri's Demon King side, which possesses the ability to actually use the demon magic that's his by birthright. He only comes out when Yuuri is really upset by some form of injustice.
- Captain Marvel is the Superpowered Alter Ego to 10-year-old audience surrogate Billy Batson. In The Golden Age of Comic Books they were definitely separate personalities. Nowadays, however, after Crisis on Infinite Earths, Marvel is a Super Mode of the same child personality who acts often in a very childlike and innocent way, but there is still some noticeable change due to the addition of the Wisdom of Solomon to the character.
- Deadman has shades of this; he's a ghost who can possess people, granting them his acrobatic skills and reflexes, and giving him whatever abilities the host may naturally possess.
- DC Comics' The Demon
- DC Comics' The Creeper, who supposedly inspired Freakazoid! (see below).
- One version of Rose and Thorn. They didn't even know what the other was doing, and had to write to each other, Rose leaving a message to tell Thorn who needed a good butt-kicking.
- The Mighty Thor does not have a conventional secret identity. Rather, he exchanged places with a mortal much like DC's Captain Marvel (above). This has been subject to occasional exceptions (such as during Walt Simonson's run), but Thor existing without his mortal counterpart never seems to last long.
- Similar to the Mighty Thor example, one time Rick Jones and Captain Mar-Vell were united by negabands, and only one could exist in the "real world" at a time, the other being transported to the Negative Zone. This meant you had a teenager apparently transforming into an older, wiser superhero named Captain Marvel. Yes, he did say 'shazam' when switching once. The writer, Roy Thomas, intentionally made the relationship between the two into a reference to the Distinguished Competition's Captain Marvel/Billy Batson relationship. How Marvel's lawyers explained it to DC is a mystery.
- A similar negaband process linked Rick Jones and Mar-Vell's son Genis-Vell, Marvel's third Captain Marvel.
- In the 1970s Carol Danvers (eventually Captain Marvel herself), would be replaced by a Kree alter ego with noticeably shorter hair and a completely different personality when she transformed to Ms. Marvel.
- Marvel Comics' Sleepwalker.
- The Incredible Hulk centers around a human who transforms into the powerful Hulk. The Hulk is portrayed as being an independent entity, and the extent to which Hulk and Banner share perceptions and experiences changes depending on the author. Some authors depict Banner as being aware of the Hulk's actions, but others do not. Some authors, such as Peter David, have attempted to combine Hulk's various personalities but these never last long. More recently, there were "team ups" between Hulk and Banner where their mental perspectives aligned for various reason, letting Hulk and Banner switch at will of the one "driving" but not combining.
- Aurora and Jean-Marie of Alpha Flight. Jean-Marie is shyer and Aurora is more butt-kicking, though to what degree each has these traits is Depending on the Writer.
- Johnny Blaze, the first Ghost Rider, was like this, originally being possessed by a demon when in his Rider form. Danny Ketch, the second Ghost Rider, also had shades of this but his was the ghost of a distant ancestor seeking vengeance. Then both of them were retconned to have their powers be angelic in nature with it zig-zagging back and forth if they had control over the powers or if the powers came with another personality.
- The Marvel 2099 version of Captain America in Secret Wars 2099 is a mild-mannered and pacifistic woman named Roberta who is completely unaware that danger or a trigger phrase can cause her to gain instant muscles and a hard but heroic personality. Her fellow Avengers (and her husband, who is secretly Alchemax Operative One-Nine-Four-One) work to keep this from her. Captain America, meanwhile, knows she turns into Roberta, but sees them as different people.
Captain America: I have difficulty with ... informality.Miguel Stone: Funny, Roberta doesn't.Captain America: Yes, well ... I'm not Roberta.
- In Rose Of Pollux's Metamorphosis, the Chameleon-arched Jamie's alter-ego, who claims the title of the Piper, can be seen as this. He's seemingly smarter and more physically hardy, but significantly less compassionate than Jamie.
- In the Buffy the Vampire Slayer one-shot You Gotta Have..., Buffy Summers isn't the Slayer. Rather, her split personality Faith (who looks the same as canon) is. Faith has some awareness of Buffy's actions but Buffy doesn't seem to have any awareness of Faith's.
- Wild Cards: Captain Trips is a renowned biochemist and a burned-out hippie, with the ability to use various drugs (usually derivations of psychoactive drugs such as LSD) to transform into several other forms, each with their own powers and individual personalities.
- The titular Captain Underpants is a Flying Brick in his superhero identity thanks to him drinking "Super-Power Juice". As his normal alter-ego Mr. Krupp, he's just an angry fat guy who doesn't demonstrate superpowers due to his identity being a product of botched hypnosis and presumably not knowing about his powers.
- Glory from Buffy the Vampire Slayer is a deity from another dimension who can only exist in this dimension by hitching onto a person born specifically to hold her in this dimension. Ben has to constantly find himself waking up miles away with no memory of what has happened due to Glory taking over his body whenever she likes.
- Heroes has a couple of characters with a power-dependent personality disorder, such as Niki/Jessica. Niki/Jessica depicted the switch between the two with the tropes of Dissociative Identity Disorder a la Primal Fear,The Three Faces of Eve and Film/Sybil, aside from a brief flirtation with the idea that this was a twist on genuine D.I.D. with Jessica being the ghost of Niki's sister.
- Diana Prince never exhibits superpowers until she transforms into Wonder Woman. The trope is somewhat averted since the source of Wonder Woman's powers in Man's World is her belt. In season one we see a few examples of her belt being taken away and her losing her powers but this never happens in season two or three.
- Kamen Rider Den-O combines this trope with a benevolent form of Demonic Possession. The normally meek main character Ryoutarou is assisted by four Imagin, which are humans from an erased timeline who only exist in a spiritual form. These Imagin have the ability to possess Ryoutarou, giving him entirely new powers. These range from Super Strength, a Compelling Voice and the power to force people to dance.
- The Ultramen from the Ultra Series are alien heroes who merge with human hosts. The extent to which they are "Alter Egos" varies from show to show.
- The original Ultraman was something of an anomaly though, as it was never completely clear to what extent Hayata and Ultraman's personalities were merged as "Hayata". The implication seems to be that it's really Hayata the whole time, but he got his memory erased as a side-effect of being brought back to life at the end of the show.
- Ultraseven, Ultraman Leo, Ultraman 80, Ultraman Mebius, Ultraman Orb, and Ultraman Geed merely take human form rather than merge with one.
- In Ultraman Ace and Return of Ultraman, Ace and Jack's respective human hosts Hokuto and Goh decided to permanently merge with their Ultras at the end of the series, meaning that the Ultraman becomes the dominant personality.
- The 90s revival series (Ultraman Tiga, Ultraman Dyna, and Ultraman Gaia) mostly do away with this by having the human host in full control over their body.
- In Ultraman Geed, Ultraman Zero presents an interesting case as he can take full control of his human host Leito Igaguri whenever he wishes.
- In Tales of Symphonia: Dawn of the New World, Emil gains enhanced fighting abilities when in Ratatosk Mode. Additionally, his eyes turn red and his voice deepens. However, it turns out to be an inversion: Emil is actually an Underpowered Alter Ego that Ratatosk fabricated so he could recuperate after almost being killed.
- In Tears to Tiara 2, the goddess Tanit is one to the goddess Ashtarte. Tart tells everyone Tanit is her older sister that she summons. Tanit is more powerful, beautiful, calm, tranquil, and wise.
- In Noob, the group's incompetent healer Sparadrap sometimes gets "borrowed" by his usual player's younger and much more competent brother. With the series happening mostly in Deep-Immersion Gaming, it plays out as if Sparadrap had two personalities : that of his usual player most of the time, during which he's completely useless, and that of his younger brother, who can actually use his abilities correctly, making him able to fend for himself and help his guildmates.
- Freakazoid! doesn't make it explicit, but his personality is meant to be the sum result of the Internet (fortunately it's the mid-1990s and the Internet didn't quite have the bit rate yet for its major purpose) and he flips back and forth between the superpowered nutbag Freakazoid and the very different personality of Dexter Douglas. In the first episode, Freakazoid describes himself and Dexter as "different parts of the same whole", but they're more often portrayed as effectively separate people Sharing a Body.
- In Masters of the Universe He-Man looks somewhat different from Prince Adam, but they seem to be basically the same person. The same cannot be said for Cringer's transformation into Battle Cat, where the Cowardly Lion becomes a fearless riding tiger who seems rather contemptuous of his alter ego.
- The The Avengers: Earth's Mightiest Heroes! version of the Hulk really takes this all the way. Hulk and Banner are definitely two different people and can talk to each other. As you can imagine, the Hulk is not a terribly good listener... but this incarnation of the Hulk is more sapient than most other versions and can be more reasonable than you would expect.
- The Blue Spirit from Avatar: The Last Airbender inverts this by forgoing the Elemental Powers that he uses under his Secret Identity.