Marcus Diamond is a former Baltimore police officer who was shot 3 times in the line of duty and survived with the help of Johann Maelzel's Metronome, as revealed by Mrs. Frederic in "Stand". He was able to survive the most painful deaths due to the metronome. He is an associate of Sally Stukowski. They are both employed by Walter Sykes who is working against the Warehouse in Season 3.
He was introduced in "Love Sick" when he was shown driving the young slacker/hacker responsible for unleashing the artifact-enhanced computer virus away from the scene, to a meeting with his employer.
He witnessed Sally's capture from his black car. Pete stared at his car realizing something was not right. After Pete stared at his car, he drove away and informed the mysterious man that Sally was captured. He subsequently killed Sally after she told the mysterious man about her attempts to turn Steve Jinks, apologizing to her as he did so. It was revealed he is an old friend of the mysterious man.
He informed his friend that they were unable to destroy the shackle and was shown Lok Archer's body. Walter told him that Archer gave him all the information they need and was ordered to recruit Steve Jinks.
In the following episode, he met Jinks at a bar. Even though Jinks was reluctant at first, he decided to join Diamond and Sykes.
In the 1st part of the Season 3 finale, Jinks went with him to capture H.G. Marcus met Pete, fought with him, and fell 5 stories. However, he lived because of Johann Maelzel's Metronome. At the end of the episode, he killed Jinks.
In the 2nd part, he went to Leena's Bed and Breakfast and attacked the Warehouse with Black Bart's Cannon, which survived thanks to the Remati Shackle. He held Leena hostage until Mrs. Frederic stopped him. He died when Claudia stopped Johann Maelzel's Metronome that was keeping him alive.
In season 4, "Lost and Found", Marcus makes a brief reappearance at the warehouse, chasing Claudia and apparantly bronzing Steve. However, this was revealed to have been an illusion created by an artifact that was slipped into Claudia's pocket.
It has been the topic of much debate among WH13 fans as to how Johann Maelzel's Metronome was used to bring Diamond back to life. The dilemma is that the metronome's downside is that the person who used the metronome receives all the pain that the person brought back would otherwise experience; if Walter Sykes had used the metronome to bring Diamond back to life that would mean he has been feeling Diamond's pain all this time. The WH13 fan base has concluded that either Sykes has been secretly feeling Diamond's pain off-screen, Sykes tricked someone else into using the metronome on Diamond and this character has simply never made an appearance or been mentioned, or as in the case of neutralizing the downside of Ferdinand Magellan's Astrolabe by use of Francesco Borgia's Dagger, Sykes used some kind of unknown artifact to neutralize the downside of the metronome.