Light Side, Attacker, Rebel
BASIC: Veteran's Shot
Deal Physical Damage to target enemy and Expose them for 1 turn. If the enemy is Empire, also remove 50% Turn Meter. If all other allies are Rebel, grant the least healthiest ally Stealth for 1 turn. This ability cannot be evaded.
SPECIAL 1: Rebel Tactics (CD 3)
Target other ally Taunts for 2 turns and gains Defense Up. All other allies gain Stealth and Offense Up. Then, call all Stealthed allies to assist, dealing 30% less damage.
SPECIAL 2: Disguise (CD 5)
Target other ally gains the Disguised buff until Rex's next turn. The cooldown of this ability is reset whenever any ally falls below 50% Health.
Disguise (unique buff): This unit cannot be damaged or targeted by enemy attacks, and is immune to debuffs. However, it has its speed set to 0 and cannot be called to Assist, gain bonus Turn Meter or counterattack. This buff cannot be dispelled or prevented.
Rex has +20% Counter Chance and +20% Offense for each living enemy and each dead ally (doubled for Empire enemies and Rebel allies). (ZETA) Whenever Rex takes a Critical Hit from an Empire enemy, he gains Critical Hit Immunity for 2 turns and recovers 10% Protection. Whenever any enemy is defeated, all enemies lose 100% Turn Meter and Rex gains +10% Speed (stacking) until the end of the battle.
By the time of the Rebels finale, we see Rex fight on Atollon, Lothal and a few minor battles. He is shown to be very precise in his shooting and is known to be a battle strategist (One Last Battle shows this, as does Family Reunion and Farewell). This is why I made his Basic unavoidable and also have a few side effects (expose, stealth, TM removal) on certain criteria and his 1st special basically what he had Kanan do in One Last Battle.
As for "disguise": I made it a sort of tradeoff. Rex takes that character out of the equation for 1 turn completely, which makes it possible to heal that character, or otherwise keep it from dying. It was based on the numerous occasions in Rebels where Rex disguises as an Imperial stormtrooper and (spoiler) also when he does that in ROTJ.
For his Unique, I made it very anti-Empire (Rex is very much anti-Empire) and similar to Chirrut's Indomitable Will (counter chance and health steal per enemy, doubled for Empire). In Rex's case I made it counter chance and Health Steal.