We could all comment on how Bruce Banner and Hulk have been through a-lot when it comes to the entirety of their time spent with the Avengers. Bruce had this own troubles to deal with when it came down to his haunting past and personal retribution; and let's get this straight from the beginning, Banner never wanted to be an Avenger. He was brought in to research the Tesseract because of its emittance to
gamma radiation and then...things just got intense. For someone who was constantly on edge based on their anger and stress, this was one of the most important story arcs that encounter mental wellbeing and battling monsters from within.
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