Arex, an Edosian (Triexian) possessed of his tripodal species' characteristic orange skin and six limbs (three arms and three legs, with three digits on each hand and foot), began serving as a navigator aboard the U.S.S. Enterprise on stardate 5371.3 (2269).

Later that year, on stardate 7403.6, the versatile Lieutenant Arex manned the bridge science station, from which he scanned the surface of Delta Theta III in search of Captain Kirk and Commander Spock, who had both briefly gone missing among the planet's tribal reptilian aborigines. In addition to his many bridge-officer skills, Arex possessed some musical expertise, playing a kind of lute during his off-duty.

Supplemental biographical info:

Arex has been in Starfleet for many years. He is known as the best navigator in the fleet. Before he entered the service as a technician, he spent 10 years in the merchant space fleet. Arex's people are long-lived, and Arex knew from the very beginning that he wished to spend his long life in space. While Arex is an officer, and a good one, he is not a product of Starfleet Academy. He worked his way up through the ranks, becoming an officer via a "field commission" during a battle involving the Klingons and the small cruiser aboard which he was then serving. All the officers were killed in the fray, and Arex took command to effect a temporary retreat and then a return action which took the Klingon ship by surprise and allowed Arex to disable her and bring her in to face Federation charges. It is not fair to say Arex has never been to Starfleet Academy — he spent two years there some time ago, as a space navigation instructor. One of his high-scoring students was a young Russian named Pavel Chekov.

His uniforms are specially tailored for him, and though he is a rather strange sight in motion, he is not at all clumsy. His reflexes are quick, and he can move with great speed when necessary. In an emergency, he can operate the complex helm-navigation console by himself.

Arex is a thoughtful being, given to long silences. There are times when he retreats to the solitude of his quarters to deliberate ... or perhaps to worship the many gods of Edos. One never knows, for Arex never speaks of these moments of isolation.

Arex has no brothers or sisters. In fact, it is highly unusual for there to be more than one child per family on his home planet Edos. The population must be kept down due to the longer life factor. As a result, all children are precious, and families are close. Ancestral records are kept in great detail, dating back for centuries. Arex's line can be traced for 3000 years.

Aboard ship, Arex often keeps to himself. However, when the mood is right, he can be persuaded to play a flute-like instrument called a sessica. (It is native to his planet.) While the music may be lively or romantic or plaintive, all of it relates to story-songs which tell the history of his race or his family. Arex also can be persuaded — at very rare moments — to display a special proficiency in sleight-of-hand, a trick he picked up while in the merchant space service. With three hands, sleight-of-hand becomes very interesting indeed!

While Arex appears to be a quiet, efficient junior officer, Captain Kirk knows the lieutenant is entitled to wear three decorations for bravery and has a Starfleet Citation for Conspicuous Valor. Arex himself would never mention them.