A special effects engineer who was fired by Tony Stark, and who built a hero for the world to believe in because the world was ready to believe in one.
Quentin Beck is the first character in these films to weaponise the audience. He is an illusions specialist who worked at Stark Industries, and he arrives in a world that has just watched half its population return and would very much like a straightforward hero. So he provides one, complete with an origin from another universe, and creates the monsters he is seen defeating.
What makes him land is that the film gives him a crew: engineers and designers Stark also discarded, standing in a warehouse admiring their own work. It is the most convincing depiction of resentment in the series.
Minor spoiler
Beck dies, but he has already arranged the rest. His edited footage frames Spider-Man for the attack in London and reveals Peter Parker’s name to the world, which is the thing the next film has to spend its entire running time undoing.