Robert Bruce Banner, M.D., Ph.D is a renowned scientist who was highly respected for all his work in the fields of Biochemistry, Nuclear Physics, and Gamma Radiation. Having been tasked by General Thaddeus Ross to find a way to recreate the Super Soldier Serum which had originated Captain America, Banner conducted an experiment with Gamma Radiation. However when an exposure to high levels of Gamma Radiation instead of Vita Radiation went awry, the mild-mannered scientist had found that when angered or provoked, he would then transform into the huge rage-fueled and nearly mindless green-skinned monster known as Hulk.
Fearful of the damage that Hulk could inflict upon innocent lives, 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 try and take advantage of his ability, while also communicating with Samuel Sterns to cure himself. However when General Ross had sent a team led by Emil Blonsky to apprehend Banner, he was then forced to return to New York City where he recruited the help of Betty Ross and Sterns to cure him of his own curse. However, not long after this test was concluded, Blonsky himself had overdosed on Gamma Radiation and transformed into the Abomination, forcing Ross to allow Banner to turn into the Hulk and battle him before then allowing the Hulk to escape.
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. Banner was tasked with tracking down Loki using the Scepter and tracing down the radiation, working alongside Tony Stark to achieve this. Despite almost causing a disaster when he was forcibly transformed into the Hulk by Loki, Banner rejoined the team during the Battle of New York and assisted them in defeating Loki and his entire army. Once the Avengers split up in the wake of the battle, Banner had become good friends with Stark and was also given the freedom to live in the world without being hunted by General Ross.
In the wake of the HYDRA Uprising, the Avengers regrouped once more to battle the ever growing threat of HYDRA, successfully capturing Baron Wolfgang von Strucker and retaking the Scepter. In order to make it so that the Earth could always be safe without constantly needing all the Avengers to protect it, Banner assisted Stark with 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. In his exile, Hulk somehow left Earth and crash landed on the planet Sakaar, where the stressful environment prevented him from regressing back into Banner, causing him to remain in Hulk form for two years.
On Sakaar, he became a gladiator for Grandmaster and was soon revered as a beloved champion, choosing to turn his back on the Earth and enjoy the life of a warrior. However, when Thor arrived on the planet, Hulk fought against his old friend in the arena. In the wake of their fight, Hulk learned that Asgard was under threat due to Hela's return and helped Thor and the other Revengers escape the planet, reverting to Banner in the process. In order to defeat Hela, Asgard was destroyed during Ragnarök, so Banner joined the surviving Asgardians, including the redeemed Loki, under the rule of their new king, Thor, on their quest back to Earth.