The existence of Fat Thor in Avengers: Endgame demanded the transformation of Bruce Banner to Smart Hulk. Joe and Anthony Russo's culminating film from 2019 featured several plot twists and surprises. Marvel Studios did a great job keeping its story details tightly under wraps with no leaks. But arguably two of the boldest creative decisions in the movie involved Thor and Hulk.

After their team-up in Thor: Ragnarok, they were the first two Avengers who knew about Thanos' attack. Their paths diverged in Avengers: Infinity War as Banner was sent to Earth to warn their fellow heroes about the Mad Titan while the God of Thunder went on a side-mission with Rocket Raccoon and Teenage Groot to acquire the Stormbreaker. Eventually, they were reunited in Wakanda which was where the final stand against the villain took place. As they catastrophically lost and the decimation was accomplished, the rest of the surviving heroes including the pair went their separate ways only to reunite five years later in Endgame.

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Each Avenger spent the time differently, but they mostly stayed the same when they reconvened for Endgame's time heist, unlike Thor and Banner. Lost in his guilt over not stopping Thanos and grief over losing his family, the God of Thunder checked out of his responsibilities. His mental health struggles also affected his physical being, causing him to gain a significant amount of weight. Meanwhile, Banner used his time in a different but equally drastic measure, successfully melding both of his identities to achieve the balance of Smart Hulk. While the two heroes' respective arcs played out separately from each other, they're subtly connected. As the only two who were readily available to do the Reverse Snap and survive it, at least one needed to have the cerebral fortitude to properly execute it. As such, despite Fat Thor still being strong enough to withstand the power of wielding all six Infinity Stones, he didn't have the right mindset for it. The responsibility fell on Smart Hulk, but it's also worth noting that if Bruce hadn't been able to transform himself into Smart Hulk, then he also wouldn't have been a viable candidate.

Hulk snaps the Infinity Stones in Avengers Endgame

Given this, it was imperative for the filmmakers that if they were moving forward with the Fat Thor arc for the God of Thunder, Hulk needed to have the brains of Banner, hence Smart Hulk. Admittedly, there are still a lot of questions with regard to how Bruce Banner and Hulk were able to maintain this homeostasis. But the movie simply had no time to properly tackle the subject given how much it needed to cover to satisfyingly close out the Infinity Saga. Despite that, the story demanded Smart Hulk existed in the movie, otherwise, things would've played out differently leading into its third-act battle. If both Fat Thor and the original Hulk had not been up to the task of doing the Reverse Snap, another hero would have needed to sacrifice themself just to undo the Decimation. This would've left Iron Man's final Snap less impactful and almost redundant.

While it appears Thor will have outgrown his Fat Thor phase when he returns to the big screen in Taika Waititi's Thor: Love and Thunder, there's no going back when it comes to Smart Hulk. The character is expected to appear in She-Hulk, but beyond that, there's no word where else he can pop up. But while Avengers: Endgame justified this seemingly rushed and permanent transformation for the green rage monster, Marvel Studios still needs to properly reveal how exactly Banner created this ultimate version of himself and his primal alter-ego.

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