Doctor Robert Bruce Banner is a renowned scientist in the fields of Biochemistry, Nuclear Physics, and Gamma Radiation. He is one of the scientists who tried to recreate the Super Soldier Serum which originated Captain America decades before, but however, when an exposure to high levels of Gamma Radiation instead of Vita Radiation went awry, the mild-mannered scientist found himself plagued with a peculiar condition. When angered or provoked, he would transform into the rage-fueled and nearly mindless, green-skinned monster known as the Hulk.
Fearful of the damage that Hulk could inflict, Banner chose to live a discreet life in remote parts of the world, working to cure the sick and help the poor while trying to elude those who would take advantage of his ability to change into the enormous green menace.
When a mounting threat called for Banner's specialized scientific expertise, S.H.I.E.L.D. Director Nick Fury recruited him, knowing full well that the incredible strength of Banner's alter ego, Hulk, would be an additional asset to the Avengers. After the Avengers split up, he became good friends with Tony Stark and was also given the freedom to live in the world without being hunted down.
After the HYDRA Uprising, the Avengers regrouped to battle the growing threat of HYDRA. To make it so that the Earth could be safe without constantly needing the Avengers to protect it, Banner assisted Stark in creating Ultron, an artificial intelligence. The AI quickly turned against them, trying to cause the extinction of humanity. Hulk helped in the fight against Ultron and decided to go off the grid after the team successfully defeated him.