Warning: contains spoilers for Incredible Hulk #42!
Like the Hulk, Samuel Sterns' life was changed by a gamma accident but in his case, the exposure to gamma radiation transformed the janitor into a green-skinned, deformed yet hyper-intelligent supervillain calling himself the Leader. Originally fascinated by the Hulk, the Leader has become one of the Green Goliath's longstanding foes as his plans to enslave, destroy or control the world have been repeatedly foiled. Although his place in the MCU appears to have fallen by the wayside since his introduction in 2008's The Incredible Hulk, he continues to play a prominent role in the comics, recently receiving a terrifying upgrade that makes him feel more like a true monster than ever.
Although Sterns' plans occasionally caused him to cross paths with other Marvel heroes, he has primarily focused on making the Hulk and his allies' lives hell. Much like the Hulk himself, the Leader was no stranger to death and resurrection and in the Immortal Hulk series, he has sought to understand the Below-Place and control the Green Door, the source of the Hulk and other gamma-radiated characters' resurrections. The Leader has used the Green Doors to control other gamma heroes like Rick Jones, Doc Samson and even the Hulk himself, recently dragging Bruce Banner down to the Below-Place to torture and study.
In Immortal Hulk #42 by Al Ewing with art by Alex Lins, Adam Gorham, Rachel Stott, Joe Bennett and others, the Leader has hooked up Banner and the remains of the Devil Hulk to mysterious plant-like entities, seeking to turn Bruce into a conduit for the One Below All to channel gamma energy. Despite Sterns' incredible intelligence, he can't get the hellish machine to work, unlike his devilish predecessor, Brian Banner. Haunted by visions of Brian's spirit, Sterns learns that the reason he was able to make it work was due to being possessed by the One Below All, something that the Leader neither wanted nor anticipated. When the sinister entity reached out to do so, his cries for help were ignored. Moments later, the Leader's face splits open in a grin, his eyes burning holes as he declares, "Now I see."
Considering the Leader's history with the Hulk, expectations were high for his MCU debut where he was portrayed by Tim Blake Nelson in The Incredible Hulk. He was a cellular biologist working with Bruce Banner to try to find a cure for the Hulk. Although he was able to find a temporary antidote, Banner was shocked to learn Sterns had conducted experiments with his blood samples and sought to further utilize it for other scientific purposes. When Bruce was captured and Sterns was forced to help Emil Blonsky become the Abomination, he was injured during Blonsky's escape and some of Banner's blood dripped into an open cut on his head, mutating him to match his comic book counterpart. He was last seen in Fury's Big Week, a comic book prelude to The Avengers movie that saw Natasha Romanoff come across a mutated and hyper-intelligent Samuel Sterns whose attempt to bribe the Black Widow got him shot and later incarcerated to be studied by SHIELD.
Although he was introduced and set up to become the Leader in other movies, this version of Samuel Sterns got lost in the early stages of the infantile MCU and appears to have been largely forgotten ever since. But since the Abomination is confirmed to be appearing in the upcoming She-Hulk series, there is hope that the Leader will re-introduce himself as the true threat he is in the comics. The Leader is not to be underestimated, and now that his body is the host of a powerful and dangerous entity unlike anything Hulk or the Marvel Universe has ever seen, proving that his underutilized status in the MCU is a missed opportunity waiting to be corrected.