Leader

The Leader is a massive-headed supervillain and an enemy of the Hulk.

The Avengers: Earth's Mightiest Heroes
Samuel Sterns was working as a janitor in the lab where Bruce Banner was working on a gamma project, where Sterns was exposed to gamma radiation that turned weak but brilliant scientist Banner into the Hulk. As a result of the radiation, the low-functioning Sterns was transformed into a superhuman genius. He was incarcerated in the Cube, a prison for radioactive supervillains, but after a series of mysterious breakouts, he took it over with the help of Abomination and Absorbing Man and turned it into his own base of where he made use of the research to further his goals. After a successful test of his "Gamma Dome" the Leader left the cube and escaped with his fellow Gamma mutants.

Using the Gamma Dome, the Leader planned to cover the Earth in Gamma Energy, causing anyone exposed to it to become Gamma-based mutants. Unfortunately, it couldn't affect Thor (who was a magic-based god, and therefore was unaffected by mortal science or illnesses) and Doc Samson (who had already been exposed to it). Though the generator at the Cube was destroyed, Leader had another one stationed in Las Vegas. As the Gamma Dome expanded, everyone in the city were turned into Gamma Monsters including the Avengers and S.H.I.E.L.D. agents, putting them all in the Leader's control.

When Thor and Doc Samson had been taken prisoner, Leader allows Abomination fight Thor after realizing Thor's immunity to the mutations. When Thor does regain Mjolnir, Leader summoned Absorbing Man who ended up absorbing the properties of Mjolnir, mockingly saying that he planned for it. As Thor began falling into a losing battle, the Hulk and Hawkeye (immunized from the effects of the Gamma energy) came crashing through the floor.

Unfazed by their intrusion, Leader had Abomination attack Hulk while he and the Gamma-powered Avengers attacked Hawkeye who launched special arrows that cured the infected Avengers before fighting The Leader. The Leader then summoned a battlesuit from his command board to fight Hawkeye. After Hulk had defeated Abomination by sending him flying out into the desert, Leader's battlesuit was shattered by the now virtually indestructible but defeated Absorbing Man as Hulk lifted the Gamma Generator off the roof causing the Leader's head to expand as he threw it into space. The next day, Leader and Absorbing Man were taken into S.H.I.E.L.D. custody with the other Gamma-based villains to the Prison 42 in the Negative Zone.

After the prison came under the attack of a being called Annihilus and his army of Negative Zone bugs, the Leader helped the Avengers and other prisoners in Prison 42. Due to the effects of Annihilus' presence and his Cosmic Rod, the Leader received constant signals due to his increased intelligence and ranted about voices in his head while fighting. Abomination helped him get to the control center, where he disrupted the shields protecting Annihilus. When this succeeded, the Avengers took down Annihilus, causing the pain in the Leader's head to go away, much to the Leader's joy. The Leader is not seen after this, but it is likely he was either placed back in his cell or taken to Hydro-Base.

Hulk and the Agents of S.M.A.S.H.
The Leader appears as the supplier of Annihilus' mind-control technology and the true catalyst behind Annihilus' gateway to Earth. He was the one who planted Skaar onto Hulk's team in order to learn about them in exchange that the Leader tells Skaar about his past. Skaar attempted to object, but the Leader silences him if he ever wants to learn the truth of his existence, unaware that Hulk was watching the conversation.