Arex was a Jedi training droid who spent many years with Jedi on Al'doleem, a moon in the galaxy's Mid Rim.
Arex was a Jedi training droid who served the Jedi Order on the moon of Al'doleem in the Mid Rim for many years. During the early stages of the Great Jedi Purge, Arex was present on the moon with Jedi Master Kirak Infil'a and was tasked by Infil'a with preparing the mountains defenses when something dark was approaching. When Infil'a confirmed it was a Sith, the droid ran to intercept him, but before it could, Vader tooks its weapon and threw the droid off the mountain. After Vader was thrown off the mountain by Infil'a, he landed near the droid, who had survived. Arex then told Vader that he was defeated, but Vader responded by crushing the droid, and using its parts to repair himself.
Arex ultimately survived, and between 3 ABY and 4 ABY, was among a number of droids assembled by the Droid Crush Pirates of Bestoon to take out Vader. When they confronted Vader on Ringo Vinda, Arex believed that because Vader had many of their parts installed on them, they could be able to communicate with them if they got close enough to the Sith. They attempted to carry it out, 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 then rebuilt and began to follow the Sith.
Arex first appeared in Darth Vader (2017) 3, a 2017 canon comic written by Charles Soule and penciled by Giuseppe Camuncoli.