Samuel Hayden

Samuel Hayden is the Chairman of the UAC who oversaw the Argent energy research projects at the UAC Mars Base.

Background
Samuel Hayden was born to the powerful Hayden family and was a very prodigious intellectual and philanthropist. After he was appointed as General Director of the Global Science Council, the UAC recruited him. Soon after Argent energy was discovered on Mars, Hayden subsequently took over the leadership of the UAC.

Soon after the Argent Tower was constructed in 2127, Hayden was diagnosed with an incurable brain cancer and left with a few months to live. Hayden avoided his death by transferring parts of his brain into a 3-meter tall mechanical body. It was implied that Hayden chose this form for his expeditions into Hell.

Hayden led the first manned expedition into Hell in 2145 in which he discovered the protagonist, simply called "Doom Marine". He had the Marine recovered and kept him personally under his watch away from the auspices of his disgruntled colleague Olivia Pierce. Hayden's relationship with Pierce gradually deteriorated due to her deranged, religious obsession with Hell which ultimately corrupted her.

After Pierce betrayed Hayden and planned to open the gates of Hell, Hayden awakened the Doom Marine and tried to persuade him of assisting him in stopping Pierce. He is harshly rebuffed twice at the beginning of the game, though Hayden claims to be willing to accept full responsibility for Hell's invasion.

Hayden takes the role as a supporting character throughout the game. However, he eventually betrays the Doom Marine, taking the "crucible" artifact from the player to continue exploiting Hell's resources for the sake of humanity's progress before sending the Doom Marine to an unspecified location. Before he does so however, Hayden states his certainty that he and the Doom Marine would meet again. It is possible that Hayden will appear in future Doom titles, possibly as an antagonist.

Trivia

 * Despite having been diagnosed with "incurable brain cancer", Samuel Hayden had his frontal and temporal lobes transferred to his new body. It is possible that these parts of his brain were free of cancer, and that he chose to replace his whole body rather than simply replace the cancerous parts of his brain with cybernetic equivalents.
 * It is possible to see the original Human Hayden in a hallway showing various scientists.
 * It is stated in-game that Samuel Hayden choose to make his body so large and powerful (being approximately 3 meters tall) because of how dangerous he thought his job and position were.
 * He is literally immortal from aging since his body is constantly maintained by stem cell and neuro-maintaining agent, however, he is not immune from being killed.
 * Due to his cybernetic body Samuel Hayden is over 130 years old.
 * S. Hayden has a phonetic similarity to "Satan", which may indicate Samuel Hayden may be revealed to have connections to Hell too in some way.
 * He often activates the restraining chip, which he installed inside the Doom Marine armor. He starts activating after they first meet face-to-face to prevent the Doom Marine from attacking him.

Behind the scenes

 * A General Hayden was slated to appear as a secondary antagonist in Doom 3, but was cut from the game, making Betruger the main antagonist.
 * In the Doom 3 novels General Hayden is a military man rather than a scientist, and still a human. He was a subordinate of Ian Kelliher, but represented the United States Military's interest in Mars. Hayden is opposed to Elliot Swann and Jack Campbell's investigation of the Mars facility, warning them that Malcolm Betruger won't let them pull the plug on his operation. General Hayden is one of the only survivors and is rescued by Kane.