The Hulk
Doctor Robert Bruce Banner, M.D., Ph.D., is a renowned scientist and a founding member of the Avengers. Highly respected for his work in biochemistry, nuclear physics and gamma radiation, Banner was tasked by Thaddeus Ross to recreate the Super Soldier Serum that created Captain America. However, Ross elected not to inform Banner what he was creating. During the experiment, Banner substituted vita radiation for gamma radiation and administered the serum on himself. As a result, the mild-mannered scientist found that when angered, provoked or excited, his body and brain would transform into a huge, rage-fueled, primitive-minded creature known as the Hulk.
For five years, Banner remained on the run from the United States Army. He chose to live a discreet life in South America while communicating with Samuel Sterns to cure himself. However, when General Ross sent a team led by Emil Blonsky to apprehend Banner, Banner was forced to return to New York City. He recruited the help of Betty Ross and Sterns to cure him of his affliction, but was captured shortly thereafter at Grayburn College. Blonsky, who had received a variant of the Super Soldier Serum, was administered Banner's gamma-irradiated blood, transforming him into the Abomination. This new threat forced Ross to allow Banner to turn into Hulk and battle Blonsky, before allowing Banner to escape.
After a failed suicide attempt, Banner focused on helping people. However, when Loki attacked S.H.I.E.L.D., Banner was personally recruited by Natasha Romanoff to aid Tony Stark in tracking down the Tesseract. Despite losing control and attacking the Avengers on the Helicarrier, Banner joined the Avengers during the Battle of New York, assisting them in defeating Loki and the Chitauri, ending their invasion.