Theresa Errgang was a female human in Starfleet in the 23rd century. As of 2268, she held the rank of Lieutenant, and was assigned as an A & A officer aboard the USS Enterprise under Captain James T. Kirk. She was athletic and bookish, with little sense of humor. She had on occasion acted as sparring partner for Kirk in the ship's gymnasium, where the captain found her an "interesting" opponent.
During or prior to that year, Errgang was infected by a colony of the Coalescence, which used Errgang as a host in order to investigate the Ilaiyenai people of planet Sigma Niobe II. Following the end of that mission, the Coalescence colony terminated itself, leaving Errgang with a gap in her memory during the time of her infection. (TOS - Constellations short story: "As Others See Us")