Derek Robertson is a corrupt businessman and high-ranking member of the Clandestine Company, who is looking to takeover City 17 in the aftermath of the Uprising. He is the brother of the late John Robertson.
Derek Robertson's background is unknown, although he has been involved with the Clandestine Company.
After the Combine's defeat, Derek's building company reconstructed City 17 (which was named "New 17") and named it the "truly safest city to live". Derek entered politics and looked to become the new Administrator of City 17, although this was a ploy by the Clandestine Company to get into power.
Along with the Clandestine Company Derek secretly initiated different attacks and crime throughout the city, which put the public into fear. After establishing his own security force who "cleaned" up the crime, the public where in favor of Derek as their new administrator. But Nathan Johnson wasn't trusting of Derek. Despite his attempts though, Derek intended up being voted in.
Derek and his colleagues at the Clandestine Company plotted a major event to unfold in 2016 named "Endgame", in which they would take over the earth, and then the multi-verse and establish a more ruthless Combine Empire. After staging false flag alien attacks, Derek pitched his idea for a defense project which would monitor the citizens of City 17. They were in favor due to fear of possible future alien attacks. Derek manipulated City 17 and surrounding areas by building a facility known as the "Air Exchange", which added chemicals into the atmosphere, causing most of humanity to go delirious and become less than aware of his true motives.
In March 2016 Derek held a press conference that his defense system would go online in May 2016. The Resistance gained Intel that this date would be the first "phase" of the Clandestine Company's first takeover, which would see them unleashing their army upon City 17 and beyond. A second phase would then see the Clandestine Company fully tear open the multi-verse and begin an inter-dimensional invasion, with the help of assorted alien technology from the Combine Empire, Race X and the Xenian Empire.
Despite the best efforts of Nathan and his crew, the Clandestine Company managed to takeover City 17 and tore open a gateway to the Race X world and also Hell, in which they unleashed monsters and demons upon earth. The Clandestine Company locked down City 17, making it their primary base of operations and Derek was their leader. Derek and G-Woman began overseeing plans for the final phase in which they would be tearing open the multiverse.
Derek moved to a citadel set up in Gm construct and watching closely as the death portal was being created. Nathan, Dan Mason and Chell Johnson infiltrated the citadel but were captured and brought to Derek's office, and kept restrained in stalker pods. Derek oversaw City 17 being burned up and the Resistance's efforts to fight against his demon forces, and began mocking the trio about their failed efforts. But Simon Simms, Sarah Woods, Monty Johnson and Holly Hills infiltrate the office and shoot down Combine Freeman. In panic, Derek flees to the dark core and has his mind transferred inside an advisor.
While the death portal began warming up, Nathan battled against the Advisor Derek while his allies worked out a way to destroy it. They came across the Borealis which was located underneath the citadel and being used to open up the multiverse. Although they disabled it, Derek was still persistent to destroy all of reality with his death portal. Once getting his friends to evacuate the citadel, Nathan wounds the advisor Derek and launches orbs at the machine and destroys it. The death portal is destroyed, which closes off the multiverse but tears apart the citadel, absorbing Derek and Nathan, killing them both. In his final moments Derek warns Nathan that his succession in stopping the Clandestine Company has opened up more threats to humanity, and that one day the Combine will work out a way to come back.