Super-Powered Alter Ego

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 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.

Anime and Manga

 * ULTRAMAN, ULTRAMAN! (Seven).
 * 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.  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 titular 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 and intricacies of gaming, and confidence to match. Oh, and he can rape people's minds with his forehead eye.
 * Is "mind crush" your answer to everything?

DC

 * 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).
 * Rose and Thorn.

Marvel

 * 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.
 * The writer, Roy Thomas, intentionally made the relationship between the two into a reference to the Distinguished Competition's Captain Marvel/Billy Batson relationship.
 * 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.
 * Aurora and Jean-Marie of Alpha Flight.
 * Johnny Blaze the first Ghost Rider was like this originally being possesed 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 vengence. Then both of them were retconed 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 personaility.

Dark Horse

 * This is the shtick of The Mask alongside a Mask of Power.

DC Thomson

 * Negative Continuity means they go back and forth on it a bit, but generally speaking The Dandy's Banana Man is portrayed as having a notably seperate personality from Little Eric, the schoolboy who transforms into him by eating bananas. For one thing, Eric occasionally seems to be quite intelligent.

Live Action TV

 * Heroes has a couple of characters with a power-dependent personality disorder, such as Niki/Jessica.
 * 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. 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.

Tabletop Games

 * Rich Burlew (of The Order of the Stick fame) created a base class, the Champion, that applies this trope to D&D 3.5.

Video Games

 * 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.

Western Animation

 * 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...