Rex is an older character that has managed to not only stay relevant but prominent through multiple metas thanks to his excellent leadership ability and cleansing utility.
In addition to Squad Discipline and Brothers in Arms, Rex has his basic, Impeding Shot, and Subdue. At first glance, these abilities seem to provide Rex potent damage and offensive utility. However, thanks to his relatively low damage and potency, neither of these abilities end up being particularly impressive in practice.
Rex does maintain a base speed of 140, which is just on the cusp of the upper quartile for characters that are relevant in the meta. This is particularly helpful as even if he is Dazed, he can quickly get a turn to cleanse himself.
He is not especially durable however, and will likely be the squishiest character on your team.
Clone allies gain 20% Max Health, and other allies gain half that amount. In addition, whenever an ally suffers a Critical Hit, all Clone allies gain 15% Turn Meter, and other allies gain half that amount.
Rex is one of the top arena leads right now, and for good reason. The meta’s currently infested with characters relying on AoE attacks. Lots of AoE attacks means lots of crits, and Rex converts that into turn meter for his team.
Speed is king in arena (and has been for awhile). The handful of leads that compete with Rex (Darth Maul, Darth Vader, Qui Gon Jinn, etc) all have turn manipulation of their own. Rex’s unique strength, however, is that he can provide turn meter to any character. Most of the other popular leads only are able to apply turn meter to characters that share their faction tags.
This means that you can, and should, mix and match the best characters underneath a Rex leadership.
While Rex is a top-tier lead, he can be used outside the lead role. His utility drops significantly, shifting his role essentially into relying entirely on Squad Discipline to be useful.
Still, even relegated to a single ability, Rex is a solid utility character. Ideally you’d want Chirrut or General Kenobi, or one of the passive cleansers, but Rex will do in a pinch.
Basically every team needs a cleanse, and thanks to Rex’s leadership, Rex works with well with just about every other character.
That being said, Rex still has certain weaknesses that need to be covered. Rex’s relative squishiness means you should have at least one strong tank, preferably two. The standard tanks apply here: Baze, General Kenobi, Shoretrooper, Stormtrooper Han.
His lack of damage output means that the other characters on your team will need to make up the difference as well. He pairs well with attackers that rely heavily on their special abilities to be useful, such as Darth Nihilus, Boba Fett or Rey. They’ll appreciate being kept free of debuffs (especially ability block).
Rex is one of the easier characters mod, if only because he has few useful stats. Roughly, the stats you should be looking for are health, protection, speed, and (if you can get a lot of it) tenacity.
For sets, ideally I’d suggest 4x Speed and 2x Health. You can substitute 2x Tenacity, but only if you’re able to stack it to a significant number.