Arex existed as a Jedi training droid and spent a multitude of years alongside the Jedi order on Al'doleem, a moon situated within the Mid Rim section of the galaxy.
As a Jedi training droid, Arex provided service to the Jedi Order for numerous years on Al'doleem, a moon located in the Mid Rim. During the initial phase of the Great Jedi Purge, Arex was present on the moon with Jedi Master Kirak Infil'a. Infil'a assigned him the task of preparing defenses in the mountains due to the approach of something dark. Upon Infil'a's confirmation that it was a Sith, the droid moved to intercept, but Vader disarmed it and hurled it off the mountain before it could act. After Infil'a knocked Vader off the mountain, Vader landed close to the surviving droid. Arex declared Vader's defeat, but Vader responded by crushing the droid and utilizing its components for self-repair.
Despite this, Arex survived. Sometime between 3 ABY and 4 ABY, he was among a group of droids gathered by the Droid Crush Pirates of Bestoon with the intention of eliminating Vader. When they confronted Vader on Ringo Vinda, Arex was under the impression that the shared parts between them and Vader might enable communication if they approached closely enough to the Sith Lord. They attempted this strategy, but then began to attack the droids attacking Vader, with ZED-6-7 revealing that it had been a setup all along. The destroyed droids were subsequently reconstructed and began to follow the Sith.
The first appearance of Arex was in Darth Vader (2017) 3, a canon comic from 2017 authored by Charles Soule and with art by Giuseppe Camuncoli.