- "I'll send you the driver. We've trained him with PE2 in our labs, he shouldn't be any trouble."
- ―A member of Abstergo Industries in a letter to an associate.
William Greer (22 September 1909 – 23 February 1985) was an agent of the United States Secret Service, best known for having driven President John F. Kennedy's automobile in the motorcade through Dealey Plaza in Dallas on November 22, 1963, where the president was assassinated.
Greer was secretly a member of the Assassin Brotherhood, part of the Bloodstone Unit of Boris Pash, which he joined as a soldier during the Vietnam War before being promoted to the Secret Service. The Unit was a cell of Assassin extremists, who were in fact the ones who killed Kennedy in order to steal his Apple of Eden for Pash's experimentations.
In January 1963, Greer was an officer serving in Vietnam as part of the Bloodstone Unit, an US military team working for the CIA which secretly was only composed of Assassins recruited by Boris Pash. Greer was eventually promoted to the Secret Service, but accomplished one last mission before leaving the country. While escorting CIA Agent Cooper on a covert mission, the Assassins kidnapped him and brought him to Pash, who interrogated the Templar in order to learn the plans of the Order and their puppet, President Ngô Đình Diệm, for the country.
Several months later, the Bloodstone Unit infiltrated the Templars with Greer being recruited by Abstergo Industries as part to their plan to kill Kennedy and recover his Apple of Eden. Having been trained using "PE2", his orders were to steal Kennedy's Piece of Eden in the chaos immediately following the assassination. He was also assigned to slow the vehicle down so the shooters could easily fire on Kennedy.
However, the Bloodstone Unit highjacked the plot of the Templars, the shooters being in fact Greer's teammates, with the intent to bring the Apple to their leader, Boris Pash. As Greer was leaving the scene after stealing the Apple in the confusion, he was intercepted by Alekseï Gavrani, a fellow Assassin who broke from the Unit in Vietnam after realizing that Pash was going too far. After Greer failed to convince him that Templar agent Lee Harvey Oswald was behind the assassination, Gravani bested his former teammate and took the Apple to Bill, a high ranking Assassin who like Gravani believed that the Unit was becoming a danger to the Assassin's Creed.