Clone captain Rex was a hero of the Clone Wars, and a hero of the Galactic Civil War. It turns out he didn’t retire right away after the fall of the Empire, either.
A new book written by Marc Sumerak, The Secrets of the Clone Troopers, is written as an in-universe reference book, written from the perspective of Rex. It’s the latest in a series of similar books, following ones about the Jedi (written by Luke Skywalker), the Sith (written by Emperor Palpatine), and the bounty hunters (written by Hondo Ohnaka). And in this new book, there’s a comment from Rex that reads:
“The war against the Empire may be over, but a good soldier knows that there’s plenty of work left to be done after the last shots are fired. My rebel friends and I are still picking up the pieces, but as far as we can tell, Palpatine is gone for good and the galaxy is finally free.”
Palpatine, of course, was not gone for good, since he somehow returned, but Rex obviously wouldn’t know that at this point in the timeline, set after the end of the Galactic Civil War (in which he served as the captain of the 501st legion under the command of Jedi General Anakin Skywalker). He was a hero in the fight against the Empire, as told in Star Wars Rebels. After the fall of the Galactic Republic and Order 66, Rex ran an underground network of clones pushing back against the Empire by fighting to save their brothers. At some point after this, he retired to a quiet life on Seelos, but a small rebel cell was sent by Rex’s old friend, Ahsoka Tano, to bring him back to the fight. Rex joined this rebel cell, the Phoenix Squadron, and particpated in many battles during the war – including the decisive victory in the Battle of Endor.
This, though, is the first time we’ve had a canon source hint at Rex’s continued involvement after the Battle of Endor and the fall of the Empire at the Battle of Jakku. It was assumed, given Rex’s character, but now it’s confirmed that “my rebel friends and I are still picking up the pieces”. It sounds like he’s still pretty actively involved, and that makes sense. It especially makes sense given the fact that Ahsoka Tano is such a major player in this post-Jakku timeline that’s unfolding in Star Wars television (and, soon, the big screen), with Ahsoka and The Mandalorian. I think the odds of Rex showing up in one of these projects, at least in a small way, seems very high. Dave Filoni is helping to oversee it all, he loves these characters, it would make sense to bring him in, they’ve already had Temuera Morrison connected to these shows too, and Rex had a cameo appearance in the flashback scenes in Ahsoka already.
By that point in the timeline Rex would be quite advanced in years, given his accelerated aging as a clone, which would probably make a major role unlikely. But an appearance or two? I’m guessing that will happen sooner or later.