Arex Na Eth was a Edosian (Triexian) male, one of the first to serve in Starfleet in the 23rd and 24th centuries. He was born on Triex, one of twelve male and thirteen female children.
In the late 2260s into 2270, Lieutenant Arex served aboard the USS Enterprise, under Captain James T. Kirk, as navigator.
By 2283, Arex had been promoted to commander, and wore corresponding rank insignia as late as 2293, but had stepped down to the rank of lieutenant at some point prior to 2305.
Arex was then stationed on the USS Enterprise-A under Capt. Kirk in 2286.
In 2293, Arex attended James T. Kirk's memorial service in the grounds of Starfleet Academy, along with many other former Enterprise crewmembers.
In 2305, Arex was aboard a shuttlecraft that fell through a wormhole, throwing him 71 years into the future, to 2376. After spending a brief time with the Federation Department of Temporal Investigations' Temporal Displacement Division, and a trip back to Triex to witness the death and rebirth of his father, Arex was reassigned to the USS Trident, under the command of Captain Elizabeth Shelby, as security officer.