Who he is: The Hulk, a.k.a. Bruce Banner
His power: Renowned scientist Bruce Banner was attempting to create a new version of the serum that birthed Captain America. But when the experiment went awry, blasting him with a massive dose of gamma radiation, Banner developed a condition wherein, whenever angered or provoked to a certain limit, he transformed into (to quote Tony Stark) “an enormous green rage monster,” largely invulnerable to conventional weaponry, though sufficient firepower can slow him down or even briefly subdue him, should he be caught off guard. It also means Banner can’t be killed by normal means, as any attempt will simply trigger a transformation.
His story: After being transformed, Banner had to leave his partner and lover, Betty Ross, and go into hiding. At the beginning of The Incredible Hulk (2008), Banner is hiding in Rio De Janeiro, searching for a cure, before he’s hunted down and forced to return to the States, where he ends up helping the government to put down the Abomination. He then learns to control his outbursts, going off the grid to help people in his role as a scientist and doctor, until S.H.I.E.L.D. picks him up in India (in the first Avengers film) to help them locate the Tesseract, a cube of immense power, and Banner reluctantly joins the Avengers to fight off Loki’s alien invasion. He then stays on in Stark Tower, continuing his research and fighting as an Avenger alongside the others. By the time he helps Tony create Ultron (in Avengers: Age Of Ultron), Black Widow has figured out a method for calming and transforming him back into Banner. But at the end of that film, the Hulk steals a quinjet and flies off, leaving the group behind.
Played by: Edward Norton in The Incredible Hulk; Mark Ruffalo in The Avengers and Avengers: Age Of Ultron.
Currently, the Hulk is: Missing.
Where will we see him next? In Thor: Ragnarok, due November 2017.