Who he is: Ultron
His power: Ultron is an AI creation, meaning he is a super-intelligent consciousness, which possesses the ability to upload his persona into machines and computers and thereby travel via the internet. As a result, he possesses proportionate strength and physical prowess based upon the makeup and number of robotic creations he inhabits, along with any powers (flight, energy blasts) with which he has imbued them. As villains go, that’s damn impressive.
His story: Ultron was created by Tony Stark to be a peacekeeping artificial intelligence program, tasked with protecting humanity from war. Stark created him using decrypted code he found hidden within Loki’s scepter. However, Ultron almost immediately determined humans to be the greatest threat to their own survival, and attacked the Avengers, while developing a plan to evolve humanity—by wiping it out via global devastation. He attempted to build himself an organic/inorganic hybrid body, and began uploading his consciousness to it, but it was stolen and reprogrammed by Stark, becoming the humanoid known as Vision. Ultron then attempted to wipe out humanity by raising the city of Sokovia into the sky and dropping it, but was defeated by the Avengers, with Vision himself destroying the final robotic avatar of the Ultron consciousness.
Played by: James Spader
Currently, Ultron is: Wiped out, though the code that gave birth to him is presumably still on a hard drive somewhere in Stark Industries.
Where will we see him next? Nowhere soon.