Is Rex the player character in Ace Combat 8?
The original Rex is the figurehead behind the Wings of Theve legend; after he dies, the player inherits the Rex callsign and public role.
Confirmed explanation
The campaign uses that substitution to place the player under the pressure of sustaining FCU morale. References to Rex therefore need context: they can mean the original pilot or the identity later carried by the protagonist.
What is not confirmed yet
How the deception is resolved is a campaign spoiler and has not been officially explained.