Harvey was a neural clone of Scorpius found within John Crichton's head, a result of the neurochip placed in Crichton's brain during his torture at the hands of the Aurora Chair.
Harvey manifested months after Crichton escaped from the Gammak Base, appearing while Crichton was under the influence of T'raltixx. This hallucination of Scorpius encouraged Crichton to harm the rest of Moya's crew. The neural clone worked its influence on Crichton on several more incidents -- on the Royal Planet, it pushed Crichton to escape from a ship that was breaking apart, and it prevented him from killing Scorpius. Crichton began to suspect that his stress and the long term effects of his torture was driving him crazy.
When Crichton was being tortured by the Scarrans' virtual reality device, Harvey was forced to reveal himself and his true nature in order to protect Crichton's life. Harvey had specific directives to enforce -- one was preserving Crichton's life, and another was protecting Scorpius from Crichton. Finally, Harvey was intended to discover the wormhole knowledge locked within Crichton's brain. These directives helped Crichton as often as they led him into danger -- but it was seen that Harvey was able to free Crichton from Nebari mind-cleansing.
By the time Crichton had the neural chip removed, Harvey was so deeply embedded in his brain that he could not be removed. However, Harvey decided that he enjoyed life in Crichton's head. He took on a new perspective and instinct of self-preservation above all else. When Crichton was twinned by Kaarvok, Harvey was also twinned. However, "Jack" the Ancient killed one of these versions of Harvey at Dam-Ba-Da -- only shortly before that version of Crichton was also killed. That version of Crichton, however, found himself at peace without the presence of the neural clone in his head.
Crichton, as he perceives himself while under Harvey's control
The other version of Harvey, and the other version of Crichton, ultimately reached an agreement that Harvey would stand by Crichton in any situation if for no other reason than his own survival. This agreement gave Crichton more insight into Scorpius -- allowing him to see into Scorpius' memories and ultimately allowing Crichton to complete his mission aboard Scorpius' command carrier.
When Scorpius later joined forces with Moya's crew, he promised to remove Harvey from Crichton's head. In truth, he simply made Harvey dormant. After Scorpius had been captured by the Scarrans, Harvey reawakened -- new and improved and completely loyal to Scorpius. He drove Crichton to rescuing Scorpius, and attempted to make Crichton believe that Scorpius was Earth's only hope for survival.
When Crichton was crystalized by the Eidelons, Harvey was crystalized as well -- leading Scorpius to believe that the human had been eradicated. Scorpius was instantly aware, however, as soon as Crichton -- and Harvey along with him -- had been revitalized. Scorpius sought Crichton out in hopes of constructing a wormhole weapon to use against the Scarrans in the Peacekeeper/Scarran War. Harvey, sensing his grand purpose might finally be fulfilled, urged Crichton to help the Peacekeepers. In the end, Crichton constructed a wormhole weapon to force the Peacekeepers and the Scarran Empire into brokering a peace treaty. After this was accomplished, Crichton destroyed the weapon and the Ancients removed the wormhole knowledge from his head. Harvey, with his purpose fulfilled, said a final good-bye to Crichton and disappeared forever.
Source[]
Harvey is derived from Farscape.