Captain Universe

Don't you wish you were this cool? Well, actually...

Captain Universe is a Marvel Comics character, and one of the few truly unique superhero concepts. Specifically, Captain Universe was a superhero identity that kept changing hosts- by itself! The character's tagline was "The hero who could be YOU!"

To be precise, Captain Universe was actually a semi-sentient cosmic force called The Unipower, whose main purpose was to maintain the barrier between Earth and another universe in place. Secondarily, it would grant powers to single individuals temporarily so they could fulfill specific missions. Curiously, these missions ranged everywhere from saving the world to rescuing single, normal individuals.

The concept was introduced in The Micronauts comic book series. The first Captain appearing in issue #8 (August, 1979), created by Bill Mantlo and Michael Golden. (Steve Ditko co-created the character with Bill Mantlo.) It was established there that the universe the heroes came from, The Microverse, was separated from Earth's by a "space wall" that had to be supported on both sides. On the Microverse it was maintained by the "Enigma Force" that was wielded by Commander Rann, the Micronauts leader; on Earth however, there was no specific wielder for the Enigma Force's counterpart, for some reason. Instead the Unipower sustained itself by jumping from one person to another. Such a person would gain the powers and costume of Captain Universe, along with knowledge of the mission he or she had to fulfill (but no direct communication with the Unipower); afterward its completion the host would return to normal.

Captain Universe is more a plot device than a character, since it has no regular host and the Unipower itself is rarely seen speaking (this varies by writer, however.) It's mostly used to power up heroes when needed, or to give normal people a taste of what it's like to be a superhero even if only for a short while. The concept's most famous use probably was in the Story Arc where Spider-Man gained its powers (but not its costume or knowledge of where they came from, due to a freak accident) leading the hero to gain Superman-level powers during the Acts Of Vengeance Crisis Crossover, where supervillains switched enemies, and thus gave Spidey a chance against much more powerful villains than he usually battled.

The "character" has also starred in at least one miniseries.

Tropes used in Captain Universe include:

Another Dimension: Where the Space Wall and The Microverse are located.
Awesome but Temporary: Nobody can keep the Unipower for long; once the threat it intends for you to handle is neutralized, it leaves you. This is probably for the best, as a What If...? story showed disastrous repercussions in a reality where Spider-Man was able to keep it.
Blessed With Suck: Spider-Man views the powers as this - his overenhanced Spider-Sense is more a hindrance than a help; flying without his webbing makes him airsick; and worst of all, he can barely control his powers, eventually causing New Yorkers to be terrified of him. As someone who believes "With Great Power Comes Great Responsibility", having godlike powers drives him close to the Despair Event Horizon. But then, see below.
The End of the World as We Know It: Most of the time, Captain Universe appears to thwart this kind of threat.
Heroic Host: Different in that that entity bonds with a different person each time, until recently.
Humans Are Special: apparently, since saving even single lives seem to matter to the Unipower. Guess it just likes us.(Note that in at least one case, an alien became Captain Universe.)
Mysterious Backer: Nobody knows what the Enigma Force is, but it seems to be a power for Good.
Random Number God: There seems to be no pattern to how the Unipower chooses a host, the only requirement being that it's someone in the general area of whatever disaster needs to be dealt with. In some cases, children as young as infants have become Captain Universe.
Power Incontinence: When Spider-Man gained the Unipower, the transfer was damaged by an experiment he was working on involving a device that generated strange form of energy. Thus, the transfer was incomplete, and he could barely control it. Fortunately, he was hit by a device designed to counter the original device just as the Tri-Sentinel appeared, causing him to gain the full effect and use the powers with ease, right before the final battle.
Powers Via Possession: Though the Unipower doesn't really force the host to do anything, only tells him what he has to do. Presumably if you disobey, it'll just leave you.
Riddle for the Ages: While it seems to have something to do with the Microverse, the source of the Unipower and Enigma Force seems mostly unknown.
Story-Breaker Power: The Unipower-powered Spider-Man punched the friggin' Juggernaut (bitch!) out of New York City! And he also knocked the Hulk into Earth's orbit!
- Speaking of which, one story had The Incredible Hulk himself gain its powers!!
Superpower Lottery: You win it!! For a few minutes anyway.
- Though it's possible for one to lose the lottery by gaining it; for most of the arc with Spider-Man, he perceived it as a curse, barely able to control the power.
This Loser Is You: Inverted; anyone can be Captain Universe, even an old man in a coma.