Doc was a M'Tar doctor stationed at a M'Tar facility sometime in the late 24th century.

In 2372, Starfleet officer Commander Daniel Radke had a nightmare where he was being tested on in the laboratory and became conscious. Doc, who appeared human, started engaging him in conversation as he had few colleagues to talk to, but when Radke started asking too many questions, Doc knocked him unconscious again. (Star Trek: The Cantabrian Expeditions: "Catalyst, Part One")

It's not clear when this event took place, or even if this was an actual event or a representation of something different. Doc appeared to be human, but this appearance could be deceiving.
Based on the character of the same name from Buffy the Vampire Slayer.

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