|Model||Human turned Grunder Industries ADFX-02|
Rex was formerly an unknown human aircraft technician and test pilot for Grunder Industries of Earth, that was until a random storm brought him and the F-5 he was flying into Equestria where he was forced to eject from a burning aircraft and subsequently imprisoned for "destroying a library" in Canterlot. After a year or so of imprisonment he escaped and made his way on foot to Avalon's Shadow. He then worked his way up the scale of other mechanics that were there until he was the only one left in business by both undercutting his competition and delivering quality work.
Currently, Rex is the ruler of the Avalon Territory as elected so by Avalon's Shadow, which is where most of the political sway in the Avalon Territory is anyways. Married to Thunderbolt and built both children by hand, this all prior to an accident that caused him to take over another ADFX-02 he had built so he could fly. His children are Morgan and Falken.
It is also very possible, that Rex is in fact a mercenary fighter pilot who went by the call sign Pixy. Since Morgan is painted in the Solo Wing livery, there is a connection between both Rex and Solo Wing Pixy.
Rex has a tendency to be very sarcastic, and a little secretive, but he's very much down to earth and does his best to get along with everyone, even if his jokes are borderline terrible. He tends to take a relaxed standpoint to everything and anything Celestia asks him to do he very quickly delays the task or finds some way to make it benefit himself more than Celestia.