Who he is: Dr. Samuel Sterns, a.k.a. Mr. Blue, a.k.a. The Leader (maybe)
His power: Dr. Sterns is a gifted cellular biologist with a strong grasp of the effects of gamma radiation on living tissue. His exposure to Bruce Banner’s blood has also caused him to develop a rapidly enhancing intellect.
His story: A colleague of Banner, Sterns—under the alias “Mr. Blue”—is initially contacted by Banner in the hopes of securing a cure for his transformations into the Hulk. The two men meet in the flesh when Banner, now on the run with Betty Ross, arrives at Sterns’ lab, looking for assistance. The cure seems to work, but Banner is a little miffed to learn that Sterns has been replicating his blood samples and is interested in seeing its effects on other animals. After Banner and Ross are recaptured by Emil Blonsky, a.k.a. The Abomination, Blonsky forces Sterns to inject him with Banner’s blood. As predicted, the injection causes Blonsky to turn into a massive, psychotic monster, who destroys Sterns’ lab. Sterns is splashed with some of Banner’s blood in the process, and his skull begins to mutate.
In a prequel comic for The Avengers, Sterns—now clearly insane—is captured by the Black Widow, and imprisoned by S.H.I.E.L.D. as “Project Mr. Blue.”
Played by: Tim Blake Nelson
Currently, Dr. Samuel Sterns is: Floating in a tank somewhere, thinking Leader-like thoughts.
Where will we see him next? Unclear. It used to seem like Marvel was uninterested in reviving anything from 2008’s The Incredible Hulk, but given the reappearance of William Hurt’s Thunderbolt Ross in Captain America: Civil War, it’s possible we might see Dr. Sterns again some day.