Doctor James Marcus was an ingenious scientist who appeared in Resident Evil Zero. A brilliant virologist, he became one of the founding members of the Umbrella Corporation along with Oswell E. Spencer and Edward Ashford and is also the man responsible for developing the mutagenic T-virus. Ten years before the infamous Mansion Incident, Marcus was assassinated by his own subordinates who were operating under Lord Spencer's orders. However, he would be resurrected after a fashion by the very virus he created, becoming hell-bent on destroying Umbrella.
Marcus, along with Spencer and Ashford, founded Umbrella in 1967 following their discovery of the Progenitor virus in Africa. Marcus and Spencer were interested in exploring the virus' potential military applications whereas Ashford was only interested in research for the sake of science, caring not for the potential consequences. Umbrella was founded as a cover for the trio's research on the virus, drawing in money and political authority that would allow the research to continue and expand.
By 1978, Marcus had discovered the T-virus after injecting leeches with Progenitor, which in turn would lead to further research into Progenitor and the later discovery of the G-virus and creation of Umbrella's line of B.O.W.s. This remarkable find gave Marcus much attention and prestige from his fellow researchers, though Spencer, intimidated by his findings, began to grow increasingly paranoid of Marcus.
Later that same year, taking a break from his research, Marcus took two of his most promising researchers; William Birkin and Albert Wesker. While teaching them to be responsible scientists, Marcus intended to use his findings to gain a place on the company's Board of Directors. In July, before Marcus could present his findings, Spencer intervened, by officially closing down the Umbrella training facility and transferring the T-virus project's development to the nearby Arklay Laboratory.
Even though the training facility was closed down, Marcus continued to work there. During the 1980s, Marcus was all but confined to the training facility. Continuing the virus' experimentation, such as creating the first "T"-based B.O.W.s (including the Plague Crawler, the Lurker, and the Eliminator), and learning of the T-virus' potential. Soon, Marcus' isolation took its toll on him mentally, eventually using his own assistants as guinea pigs for his leech experiments.
In 1988, Oswell Spencer's paranoia reached its peak and he decided to take action. He ordered Wesker and Birkin to carry out the assassination of Marcus. USS commandos under Wesker's command stormed Marcus' laboratory and gunned the old man down. Marcus' two most trusted proteges taunted him as he lay bleeding on the floor, and as they walked out of the lab the USS troops began gathering up Marcus' research materials and destroying what wasn't needed. Marcus' body was dumped and was carried down to the sewage treatment plant where it would be left to rot. However, what Wesker and Birkin failed to notice was that at the time of the assassination, Marcus was experimenting with one of his infected leeches. During the shooting, the containment unit the leech was sealed in was shattered and it escaped, dropping off the table and into Marcus' mouth.
Over the next several years, the leech bonded with Marcus' DNA and regenerated him, mimicking not only his body but also his memories. In the end, the leech had become a clone of the original Marcus, taking on the doctor's form as he had appeared during his youth. In 1998, this Queen Leech in Marcus' guise initiated a plan to take revenge on Umbrella, destroying Spencer and his company and anyone else who stood in the way.
For further information, see Queen Leech.