Oliver Leek was James Lester's former right-hand man at the Anomaly Research Center (ARC) and (arguably) the primary antagonist in Season 2 of Primeval.
He apparently did not exist in the show's original timeline, first appearing after time was altered by the future predators trapped in the Permian at the end of series 1.
Though throughout the majority of the series, he puts on a shy and slightly dim-witted persona, he was in fact an exceptionally intelligent and manipulative sociopath with plans to rule the world using an army of creatures extracted from various time zones throughout Earth's history.
His performance was so good that Nick Cutter initially believed that he wasn't intelligent enough to set up his operation, and mistakenly assumed Helen Cutter to be its overall mastermind.
Leek showed while setting his creature on the ARC that he could easily remain calm and composed in the face of all of the death he caused, a trait which he displayed in both his real and fake personalities. Leek is only once shown to break his calm, collected demeanor when he loses his temper with Helen Cutter after she tries to give him orders rather than act as his partner.
In the show
Allegiance with Helen Cutter
Having been overlooked his entire life, and intense desire for power over others overtook Oliver, who set up a partnership with Helen and several mercenaries (including one of her Cleaner clones) in which they would send mercenaries across the temporal anomalies to steal creatures which they would mind-control using future technology (neural clamps) into creating a semi-chaotic new world order with Oliver Leek being the overall ruler of humanity.
He demonstrated his betrayal when he set a Future Predator on his boss James Lester in a (failed) attempt to kill him, though it did succeed in killing all other staff at work at the ARC that evening under his orders, and he implanted a bomb into the ARC.
He also released a giant Silurian Scorpion onto a tourist filled beach, so that it could start his plan for a new world order by killing as many people as possible and distracting the ARC's attention. After causing several deaths, the creature is captured by Stephen Hart and presumably returned to the ARC.
Having kidnapped Nick Cutter, Jenny Lewis, Connor, Abbey and Caroline and imprisoned them in his compound, Leek took a sadistic joy in watching Caroline (who he had been paying throughout the series to provide him with information on Connor's work) and Abbey fighting, and refused to stop the violence because he enjoyed the entertainment.
After he accidentally downloaded the virus that Connor had implanted onto his laptop, Leek inadvertently released all of the creatures into the compound.
Lester sent all of the ARC's soldiers after Leek when his compound was breached, so he took Nick Cutter with him as a bargaining chip, and video called Lester (after Nick had killed one of the future predators by pulling the neural clamp out of its head), threatening to set his army of future predators on Nick if he was not allowed to go free. However, Nick short-circuited the neural clamp in the predator's head, causing it to disrupt the clamps in all of the other predators. As a result, all of the future predators in the compound attacked Leek out of both vengeance and hunger, and tore him to pieces while he screamed in agony.