Moon Knight is the first MCU release to directly deal with Dissociative Identify Disorder (DID), but they have handled an original Avenger in a very similar way. Steven Grant is introduced in Moon Knight as a meek gift-shop worker, but it is soon revealed that he is an alter of Marc Spector, a brutal mercenary. The relationship between Steven and Marc is central to the Disney+ series as the two must learn to live with each other, all while serving the Egyptian god Khonshu.
The fifth episode of Moon Knight explains Marc Spector's backstory and the childhood trauma that caused Steven to be manifested as part of a coping mechanism. Portrayals of DID have appeared before in film and TV, but it has regularly been done in a lazy way, with most productions reverting to the «Jekyll and Hyde» trope rather than developing a more representative take on the mental health condition. Although Marc Spector is the first Marvel Cinematic Universe character to explicitly live with the DID, one of the MCU's most popular heroes has had to deal with the disorder since his debut.
Related: How Moon Knight's Mental Health Representation Was Important (But Faltered)
Bruce Banner's central storyline throughout his time in the MCU is how he and the Hulk co-exist. Like Spector, Banner has a traumatic origin story that caused the creation of his alter personality, in his case a failed experiment that exposed him to gamma radiation. Although Marc Spector and his alter identities don't physically change during Moon Knight in the same way that Banner does when he morphs into the Hulk, the pairs still share many similarities. Hulk and Banner notably struggle to find balance between their alters and can feel like they don't have any control over their
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