In Ahsoka, Rex made his live-action debut in the episode "Part Five: Shadow Warrior", voiced by Temuera Morrison in sequences set during the Clone Wars.

He is physically distinguished from other clone troopers by his close shaved blond hair, and he is identified by blue markings on his armor and by stylized hawk eyes painted on his helmet.

Rex is characterized as believing it is his duty to not only carry out orders but also to protect the men under his command; however, his belief in the Republic wavers over the course of the series.

Previously, the name Captain Rex was used for the error-prone rookie droid pilot RX-24 of Star Tours,[5][6][7] who replaced an earlier concept of an unpredictable and risk-taking Clone Wars veteran.

[10] To aid with the vocal performance, Baker and supervising director Dave Filoni developed a "clone triangle" that arranged the characters by personality.

"[15] The official Star Wars Databank describes Rex as a "no-nonsense, by the book" and "exemplary clone trooper, brave under fire and dedicated to the men serving under him" who must adjust to his "maverick" and "headstrong" Jedi commanding officers.

[19][20] Filoni developed an in-universe backstory for the design and felt that Rex "trying to wear the same armor" by welding together a hybrid version was a good character trait that added to his uniqueness.

Now seemingly a much older man due to accelerated aging, Rex lives with fellow clone troopers Commander Wolffe and Captain Gregor.

In the episode "The Lost Commanders", the three are sought by the Ghost crew upon instructions from Ahsoka, and they ask the troopers for help in establishing a new rebel base.

Rex leaves Wolffe and Gregor to join the Rebel Alliance and to fight alongside the Ghost crew and Ahsoka against the Empire.

[33] In the epilogue of the series finale "Family Reunion - and Farewell", it is revealed that he was promoted to commander again and fought with Hera at the Battle of Endor.

In the episode "Decomissioned", sisters Trace and Rafa Martez have a run-in with the Bad Batch on Corellia while attempting to retrieve data from a tactical droid for Rex.

He takes them to a crashed cruiser on Bracca to use its medical bay, but during their mission, one of the Bad Batch members, Wrecker's, chip activates and he attacks them.

He later returns in the episode "War-Mantle" as a hologram, where he asks the Bad Batch to investigate a distress signal sent by Captain Gregor from Daro.

Rex made his second live action appearance in the Ahsoka episode "Part Five: Shadow Warrior", during flashback sequences in the Clone Wars.

In March 2016, Filoni supported the former fan theory of Rex being the older member of Han Solo's raiding party, previously known in Legends continuity as Nik Sant, on Endor in Return of the Jedi.

[36] In August 2017, writer Steven Melching shared concept art for the fourth season of Star Wars Rebels, and it was reported as a confirmation of this theory.

[41] In 2020, Hot Toys produced a sixth scale action figure of Rex, featuring a head sculpt, which is modeled after Temuera Morrison, and a second helmet.

[44] Eric Goldman felt that the Rebels reunion between Rex and Ahsoka was a poignant moment and the emotional highlight of "Relics of the Old Republic".