Yeoman J.M. Colt was a female Human enlisted crewmember. She served in the operations division aboard the USS Enterprise in 2254, under the command of Captain Christopher Pike. She was the captain's personal yeoman, replacing his yeoman killed in a fight on Rigel VII.
When Pike was captured by the Talosians, Colt was one of six crewmembers who volunteered to beam into the Talosians' underground city to rescue him. In fact, the Talosians had read Colt's attraction to Pike and manipulated events so that only she and "Number One" transported down. All three Enterprise crewmembers escaped the Talosians alive and uninjured. (TOS: "The Cage")
Background information Edit
Yeoman Colt was played by Laurel Goodwin. According to Gene Roddenberry's series pitches and scripts, her initials were "J.M.", but their meaning was never alluded to. In fact, her name was not spoken at all throughout the pilot, and was derived purely from script or background sources as follows:
The following character biography appeared in the series proposal Star Trek is...:
The Captain's Yeoman. Except for problems in naval parlance, J. M. Colt would be called a yeo-woman. With a strip-queen figure even a uniform cannot hide, Colt serves as Captain's secretary, reporter, bookkeeper – and with surprising efficiency. She undoubtedly dreams of serving Robert April with equal efficiency in personal departments.
The note about Colt's title not being yeo-woman was included in the scripts of "The Cage", which went on to say of the character, "About twenty, she's pert and shapely, but carries herself with trained precision." Her assignment as the captain's yeoman was referred to in the scripts as being merely temporary, though this line of dialogue is not in the final version of the episode. In another example of scripted but ultimately omitted dialogue, Colt was established as having had the same training as everyone else on the Enterprise, despite being new to the ship herself.  Also, the scripts did not include dialogue from the episode's final version in which The Keeper regards Colt as having "unusually strong female drives."
The comic book series, Star Trek: Early Voyages, named her "Mia Colt". Although this might seem contradictory to her first initial being "J", her second initial was "M", and she may have gone by her middle name rather than her first.