James Marcus is one of the primary antagonists in the Resident Evil Series.
Years before the events of the first Resident Evil, he was one of the top virologist researchers of the Umbrella Corporation and a head of its personnel training facility, personally appointed by Ozwell E. Spencer to work with the Progenitor virus. In this capacity, Marcus played an integral role in developing the T-Virus (Tyrant virus), and the Tyrant and Nemesis programs, also conducting extensive research on leeches that led to the development of the G-virus, up until his death in 1988.
Spencer betrayed him by sending Marcus' direct subordinates, Albert Wesker and Dr. William Birkin, to murder him and steal his research. They dumped his body away along with the Queen Leech that Marcus created, the creature consuming his body and ended up absorbing his genetic memory as consequence. This resulted in the Queen Leech believing itself to be Marcus reborn.
Serving as the antagonist of Resident Evil Zero, The Queen Leech avenges the real Marcus by triggering the events of the first Resident Evil game by causing a T-virus outbreak in the Arklay Mansion before being destroyed by Rebecca Chambers and Billy Coen. Marcus also appears in Resident Evil: The Umbrella Chronicles.