Hulk was only the second MCU hero to get his own solo film and a lot has changed since then. The disappointment of The Incredible Hulk led to Edward Norton leaving the MCU and no more Hulk solo film being developed as of now. But the character made an acclaimed return in The Avengers, with Mark Ruffalo taking over the role.
Given all the change the character went through in Phase 1 alone, it's not surprising that Hulk has continued to evolve as the MCU went on. Even without his own solo films, he has had a fascinating arc and some great character development that has brought him a long way since the early days of the MCU.
His Look
Ang Lee's version of Hulk drew a lot of criticism for looking pretty goofy. When the MCU redesigned him, they gave him a more vicious and scarier aesthetic, really embracing the fact that this is a rage-filled monster.
However, the MCU softened Hulk's look in The Avengers and that has continued to tweak it in each new appearance. Hulk also begins to resemble Mark Ruffalo more and more until his appearance in Avengers: Endgame where it's impossible not to see a green version of the actor.
Control Of Hulk
A huge part of Bruce Banner's character is that he does not have control over the Hulk side of his personality. Whenever he gets angry, the Hulk is unleashed and Banner is lost until he calms down again.
In The Incredible Hulk, Banner is trying to control it as best he can with breathing techniques but it's fairly unsuccessful. The Avengers then introduces a new idea that Banner is always angry and Hulk is ready to come out at any moment. However, this control is inconsistent over the movies where Banner sometimes summons Hulk at will, and other times he cannot.
On The Run
The Incredible Hulk forgoes retelling Hulk's origin story and picks up with him on the run. Given his power, he is constantly being hunted by the military, led by General Ross. Not only does he have this monster inside him, but he is not able to rest ever.
Over time, Hulk becomes more of a hero and his days of running seem to be behind him. By the time he is reintroduced as Smart Hulk, he is a welcomed member of society, even taking pictures with fans.
Intelligence
Despite Banner being one of the smartest people in the MCU, when he becomes Hulk, he seems to lose a lot of that intelligence. Hulk has a very one-track mind where all he wants to do is smash.
However, he begins to grow more intelligent , especially when Hulk sticks around longer. In Thor: Ragnarok, it is revealed that Banner has been Hulk for years without transforming back. Now he can actually talk and have conversations. Then comes Smart Hulk, which is the perfect combination of Banner's brain inside Hulk's body.
Romance
One of the main plot points of The Incredible Hulk is Bruce's romance with Betty Ross. They were together before his accident and his life on the run kept them apart for years. Despite all this, they reunite and it's clear they still love each other.
Although this seems like a very significant relationship, Betty is never mentioned again in the MCU. Eventually, Banner strikes up a romantic relationship with Natasha, but that also only lasts one movie before it is forgotten.
Friendships
Hulk is not always an easy guy to get along with even though Banner seems like a really nice person. After being on his own in The Incredible Hulk, Banner forms a quick friendship with Tony Stark when they meet in The Avengers.
However, as the movies continue, he develops other relationships with his fellow heroes. He gets close with Natasha and in Thor: Ragnarok, both Hulk and Banner form a buddy bond with Thor.
Defeated
Part of what seems to drive Hulk is that he knows he is the strongest being around. He might not be smart but he knows he can win any fight. This remains true for much of the franchise until he meets Thanos.
In a shocking display of the Mad Titan's power, he easily beats Hulk up. This changes Hulk and he becomes much more reluctant to fight all of Banner's battles now that he has experienced the pain of defeat.
Secret Weapon
Though he was always a bit unpredictable, Hulk was the secret weapon for the Avengers. Whenever there was a threat that seemed more than they could manage, Hulk was called upon and suddenly the scales were tipped back in his allies' favor.
This became less of the case as time went on, and it wasn't so much because Hulk didn't want to fight anymore. With more and more powerful beings being introduced, and characters like Thor becoming more powerful, there wasn't as big of a need for Hulk.
A Tragic Character
Out of all of the heroes, Hulk has always been the most tragic character. Banner lived a miserable life of fear and loneliness. In The Avengers, he explains how when he is Hulk, he talks about trying to kill himself, and in Age of Ultron, he laments never being able to have a family.
Thor: Ragnarok allows the character to embrace his lighter side. Hulk becomes a lot goofier and even Banner is funny. This carries over into the other movies as Banner struggles with Hulk in Infinity War and is great comedic relief in Endgame.
Banner And Hulk Dynamic
The relationship between Banner and Hulk has always been fascinating. In the beginning, Hulk is a thing Banner is trying to get rid of before realizing that maybe it is just something he needs to control.
The evolution of their relationship is great as Hulk then takes over as the dominant side of their personality, before refusing to come out at all. It all leads to the final phase in which Banner and Hulk learn to coexist.