The Incredible Hulk
Bruce Banner's angry alter ego, the Incredible Hulk, was the Avengers' first quarry. The team's raison d'etre after Loki set several heroes on a collision course with Hulk, he is now one of their major foes.
Green with Anger
Physicist Dr. Robert Bruce Banner was developing an experimental Gamma Bomb at a top-secret goverment research facility when teenager Rick Jones wandered onto the test site minutes before the Gamma Bomb's detonation. Banner rushed to save him by pushing Jones into a shelter, but was himself bombarded by the full measure of gamma radiation. The even-tempered physicist's body was changed at the cellular level, imbued with gamma-irradiated power unleashed in moments of stress or anger. During those times, Banner mutated into a Green Goliath dubbed "the Incredible Hulk."
Source: Marvel The Avengers Super Hero Guide
Dr. Bruce Banner