For this idea MC is reincarnated as Hulk... not as Bruce banner but Hulk.
In the movie Hulk (2003), it was shown that his father experimented on himself which is passed on to banner and later on when he is exposed to gamma radiation hulk is born.
For this fanfic, Hulk is just another personality of Bruce, where Hulk has powers while Bruce doesn't. It is similar to Caitlin Snow and Killer Frost in Flash (tv series started in 2014). Hulk personality was born when his father tried to kill him and his mother tried to stop him which result in getting herself killed by his father.
Hulk personality remains subdued till the events of Avengers movie. All the previous encounter when bruce turns into Hulk was a mix personality of bruce and hulk but neither were in control. It can be said that both were just on instict in Hulk form. As instinct he always go after anyone who tries to harm him and as Bruce loves Betty so whenever he see her he calms down (later Bruce affection turns to Natasha after Avengers movie) or if anyone tries to harm her he attacks them all can be explained as instinct which all animals (humans are social animals) pocess.
When Bruce Banner was affected by Loki's Scepter aka Mind Stone, during the time when Loki was prisoner in SHIELD Hallicarrier, Hulk got awakened fully. Here MC takes place as Hulk.... Hulk Banner.... 2nd personality of Bruce Banner. Hulk Banner considers Bruce Banner his elder brother(because hulk was born at the time of death of their mother hence Bruce is elder) as their father considers Hulk, his actual son.
Tips on how MC can effectively use Hulk power without remaining angry:
* As shown in movie during the fight between hulk nd his father when military was monitoring their fight it was mentioned that surrounding ambient energy was being depleated.
* His father had the power to absorb the power from different power source (not ambient energy) or turn into the material he touches, i.e. taking the features of said materials.
* So it can be concluded that Hulk was taking ambient energy while his father was taking energy from hulk which get overload and due to missile he went poof.
Hulk get stronger when he gets more angry... but in reality he is just subconsciously taking more ambient energy due to anger and get stronger. MC will learn to control this and can power up according to the situation like need to fight Loki take little ambient energy or need to fight Thanos take massive ambient energy.
A/n : I don't know what sort of memories can a spilt personality retains from the original person so Author can go spit some BS or can search google and give MC extra memories of Bruce as a spilt personality (along with the memories of past life of transmigrater-MC).
Again any1 interested in writing a story using this idea is welcome to write just leave the link to story in inbox or at least in comments section.