Hulk is the form taken by Bruce Banner whenever he is stressed, enraged or seriously injured. This is due to exposure to gamma radiation during an experiment attempting to find a way of making humans immune to gamma radiation. Bruce experiences flashbacks of when he was in his Hulk form.
When Banner is upset, angered, injured or in danger, he transforms into one of the most powerful beings in the MCU, 9 feet tall, green-skinned super powerful man-beast known as the Hulk. As the Hulk, Banner possesses an nearly unlimited degree of strength, speed, durability and regenerative capabilities that increases whenever he is even angrier, hence the old saying "the madder he gets, the stronger the Hulk becomes."
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