Unknown (presumed dead)
Dr. Michaels was a scientist assigned to the Delta Labs on Mars. He was one of the few UAC personnel to survive the initial demon invasion. During the invasion, he poorly barricaded himself in his research lab and began studying the corpse of an Imp brought back from one of the portal expeditions. Dr. Michaels wore a HAZMAT suit.
When the player meets Dr. Michaels in Doom 3, he is startled by the player's sudden appearance, but is grateful to see the player, believing you to be a sort of rescue team. He explains that he's been studying a demon corpse in the hopes of finding a weakness to help combat the demon invasion. His attempts have been unsuccessful, though he attributes this to a lack of time. He states that he'll continue his attempts, and gives the player the code for a pair of storage cabinets in the adjacent room. If the player persists in speaking to him, he will state that he can't leave because of the importance of his work, and tells the player to find help.
While his fate is unknown, he was most likely killed off-screen, since Dr. Michaels' lab was poorly barricaded and the intro to Resurrection of Evil states that the Marine was the only survivor.