(Covers information from the S.D. Perry novels)
James Marcus was an Umbrella researcher. Assassinated in the 1980s, he was revived in 1998 thanks to the power of the T-virus and his Queen Leech experiment. He was later destroyed by the Queen Leech when it transformed to fight Rebecca Chambers and William Coen, destroying the remains of his body in the process.
In the Perry universe, Marcus actually appears in the story as the villain. In the Capcom universe it is merely the Queen Leech believing itself to be Marcus.