Dr. Cloud was a scientist assigned to the Erebus Facility on Mars. During the second demon invasion, Elizabeth McNeil wanted the Marine to recover a tablet called The Primitive and take it to Dr. Cloud.
When the player meets Dr. Cloud in Resurrection of Evil, he is speaking with Elizabeth McNeil through a computer screen. After scanning and translating The Primitive that the player acquired, Dr. Cloud states that his and Dr. McNeil's assumptions about The Artifact being a weapon of unbelievable power were correct. He then goes on about there being a reference to three hunters that were tasked with protecting The Artifact. He tells Dr. McNeil that he'll contact her when he knows more before she ceases communication. He then explains to the player that The Artifact is also a gateway between Hell and 'our dimension'. He tells the player to get The Artifact to Dr. McNeil, and gives the player a keycard to unlock the armory.