Lieutenant Commander Tom Markel was a 23rd century Starfleet officer in command of the survey ship USS Ariel.
In 2269, following the disappearance of landing party of three crewmembers, he and the two remaining members of the crew beamed down to the surface of Lactra VII to rescue their crewmates. They were captured by the Lactrans and were placed in a Lactran zoo until they were freed, about six weeks later, by the crew of USS Enterprise. (TAS: "The Eye of the Beholder")
Markel was voiced by James Doohan.
Despite his rank of lieutenant commander, his uniform has commander's stripes on the sleeves.
In the novelization, Markel's first name is Louis.