Like the comics, the Marvel Cinematic Universe is divided between several levels of scale. The smallest heroes clean up the streets, while the biggest save the universe. When it comes to affairs on Earth, the US government plays a pretty sizable role. Thaddeus E."Thunderbolt" Ross is one of the most important political figures in the MCU, and he's about to become even more powerful.
Most fans of the MCU will recall Ross for his role in Captain America: Civil War. He wasn't exactly the villain of the piece, but he was the loudest voice demanding the signature of the Sokovia Accords. Though he had the best intentions, his actions helped to divide the Avengers, possibly partially causing their defeat at the hands of Thanos. After five years in the grave, he's back to star in the new Cap's next adventure.
Ross was Originally a Hulk Villain
Thunderbolt Ross was introduced in the first issue of TheIncredible Hulk in 1962. He was the Air Force general in charge of Bruce Banner's gamma radiation project. When the bomb transforms Banner into the Hulk, he commits himself fully to pursuing the monster he helped to create. This obsession leads him to ally himself with a variety of dangerous supervillains. He partnered with a monster called Zzzax to gain electrical powers, only to lose his mind as a result. Eventually, he saw the heroic side of the Hulk. He sacrificed his life for his daughter and died, offering a blessing to Bruce Banner. Comic book characters never really die, so Ross consistently returned to lead an army against the Hulk.
Ross is the Red Hulk
The Incredible Hulk can't be defeated by much, but another Hulk will usually do the trick. Red Hulk was introduced in 2008 in the first issue of the second volume of Hulk. He's an intelligent crimson variant of Bruce Banner's alter-ego. Uniquely, Red Hulk's true identity was a secret to the audience as well as the other characters. He appeared in comics for two years before his name was revealed. Red Hulk is a controversial villain. He's extremely violent and powerful, but his perceived immortality can make him dull. In his first storyline, Red Hulk killed a few major villains and defeated many Marvel heavy-hitters. He was eventually defeated by the Hulk and Thor. Later storylines would see him attain the Power Cosmic, fail to take over the world, and join the Avengers. He's a strange figure that consistently changes motivation and switches teams. Many fans thought a version of Red Hulk would be introduced in She-Hulk, but he's still exclusive to the comics for now. If he appears in New World Order, he's likely to be a powerful antagonist.
He has a History with The Leader
Thunderbolt Ross will probably not be the main villain of Captain America: New World Order. Instead, that role appears to be being filled by Samuel Sterns, AKA The Leader. The Leader is the opposite of the Hulk. He worked at a chemical plant, where an explosion exposed him to gamma radiation. Sterns's skull and brain expanded to an incredible size, leaving him with an advanced intellect. Sterns has been a recurring Hulk villain with designs on taking down the United States government. During Thunderbolt Ross's mad Hulk-hunting phase, he partnered with the Leader. After Ross's first death, Leader reanimated him to replace one of his soldiers. The Leader will likely be the primary antagonist of the film, but his connection to Thunderbolt Ross raises some interesting questions. The MCU version of the Leader will be a scientist who was contaminated during the events of the 2008 Incredible Hulk film. This keeps his connection with Ross alive.
Ross is Now the President
In his first appearance, Ross is a US Army General. By the time of Civil War, he was the US Secretary of State. After the events of Endgame and The Falcon and the Winter Soldier, he's been elected the President of the United States. Fans may know that the MCU abandoned real-world presidents shortly after its beginning. In the MCU canon, President Barack Obama won the 2008 election but was defeated in 2012. Instead of Obama, the POTUS of the MCU was a man named Matthew Ellis. There's not much to be said about President Ellis, as his policies are never important to the plot. Donald Trump became the President in 2016, replacing Ellis and instituting a few relevant policies. Ross will be the first president who is particularly important to the narrative of the MCU.
Ross is Still Pushing the Superhero Registration Acts
The Sokovia Accords have been repealed, but Ross is still working to keep the Avengers in check. The most recent action of the character is his appearance at the end of The Falcon and the Winter Soldier, in which he classifies all documents relating to the "Enhanced Humans Act." After everything that happened in the battle against Thanos, after dying for five years, and after seeing his work be repealed, he's still trying to accomplish the same goal. Ross might gain power, but he rarely seems to gain knowledge.