(Capcom's primary storyline)
Lee D. was an Umbrella Corporation employee with the very high Level 9 security clearance.
With this clearance, he had knowledge of "Stairway of the Sun", the African flower linked with the Progenitor Virus production.
Lee was eliminated after the Raccoon City Destruction Incident under the orders of Oswell E. Spencer, intent on preventing the flower's existence from getting out should the company collapse.
The file Spencer's notebook containing Lee D.'s name in it, as well as many other Umbrella employees, can be found in Resident Evil 5.