Marc Brooks was a male Human in the 24th century. Born in 2329, he met and married Janet Brooks by 2367 and lived with her aboard the USS Enterprise-D.

Five months prior to stardate 44356.9, Marc died in an accident. Shortly following the funeral, Janet had all of Marc's belongings removed from their quarters. Unbeknownst to her, Deanna Troi had saved one special item, a music box, just in case Janet changed her mind. The grief over his death led Janet to seek counseling from Troi, where Deanna returned the music box to her. (TNG: "The Loss")

This character was only mentioned in dialogue.
It was never made clear whether or not Marc Brooks was a Starfleet crewmember, although the fact that Troi was present following the funeral and that Captain Picard seemed familiar with Brooks implied he was aboard the Enterprise at the time of his death.
The Star Trek Encyclopedia (3rd ed., p. 57) stated Marc Brooks was a Enterprise-D crewmember who died in late 2366.