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.
When the superpowered side is evil, then it is, rather unsurprisingly, the subtrope Super-Powered Evil Side. This trope itself is the subtrope of Sharing a Body.
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Anime & Manga
DN 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.
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.
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 with more justification, 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 didsay 'shazam' when switching once. How Marvel's lawyers explained it to DC is a mystery.
The writer, Roy Thomas, intentionally made the relationship between the two into a reference to the Distinguished Competition's Captain Marvel/Billy Batson relationship.
A similar negaband process linked Rick Jones and Mar-Vell's son Genis-Vell, Marvel's third Captain Marvel.
In the 1970s the current Captain Marvel, Carol Danvers would be replaced by a Kree alter ego with noticeably shorter hair and a completely different personality when she transformed to Ms. Marvel.
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.
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.
Negative Continuity means they go back and forth on it a bit, but generally speaking The Dandy's Bananaman is portrayed as having a notably separate personality from Little Eric, the schoolboy who transforms into him by eating bananas. For one thing, Eric occasionally seems to be quite intelligent.
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.
Live Action TV
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.
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.
Averted when the originals turn up for team-ups in later series — apparently, Ultraman through Ultraman Eighty liked their hosts' forms well enough that they'll take them when they're on Earth and need to operate quietly.
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 got his memory erased as a side-effect of being brought back to life at the end of the show.
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 incompentent healer Sparadrap sometimes get "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 himslef 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. However, they seem to share the same girlfriend...
Later incarnations of the Masters of the Universe franchise depict He-Man as being a lot taller and more muscular than his civilian alter ego, Prince Adam.
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.