The first evident clue is the Latin meaning of 'Rex', meaning King--but it's also outright stated that it wasn't the handiwork of Oric, who is, presumably, the King.
and the shoddy drilling on the compartment tells you this wasn't the work of Oric or whoever constructed the hub.
So, with him ruled out, who? Well, consider the circumstances of where it was found. First off, it's stashed away in one of the boss hubs. If it were left in a normal DL, it'd randomize and be lost before long, but instead it was placed into a sub-DL, like Proxxy's hub. It's not exactly COMPELLING evidence but who'd know the mechanics of the DL's randomization better than the Mechanic?
Proxxy also states this:
It's been hidden in a secret compartment in your hub for who knows how long
While another Sibling may have had access to the Mechanic's hub, I think it's far more likely that the Mechanic themself made that compartment and placed it there. On top of that, it uses a method of encryption that has insofar been proven uncrackable. While encryption may fall more into Ecco or Floe's territory, the Mechanic, by nature of there job, probably wouldn't be clueless. And, on top of that, it IS something left through the cycles, which seems to threaten Oric's beloved stability. The previous Voice likely would've been happy to help the Mechanic encrypt it if Ecco's rebellious nature is nothing new.
Finally, on the Memor-E Al got on cleared DL2:
You got an OricSoft MEMOR-E Cartridge!
MEMOR-E #1: SERVANT SERAPH
ORIC descends the ladder to DUNGEON LEVEL 2 to face off with his final foe: the SERVANT SERAPH. With his hard-won skills, smarts, and loot, he is prepared for the last challenge... or so he thinks. Featuring cameo appearances by REX and VI.
We know for a fact that Ciro is Vi, and as such, a lot of people seem to have latched onto the idea that Rex is another incarnation of Riltia. However, that's...Not necessarily true. While we now know that Riltia does enjoy being with the Target in various cycles, we also know that she isn't always, and on top of that, that she betrays them frequently. So, I think it's more likely Rex was a previous Mechanic. Why? The Mechanic reigns over DL2; this is the closest floor to the Target's native floor (Evidently), it's where the Memor-E was found (And it's very very clear that clearing a DL somehow makes things from previous cycles come over; take a look at, well, pretty much ANY of the monsters fought there), and finally, Oric is descending to DL2.
Also, I kind of wonder...Is it possible that the Sinister Siblings, or anyone with a soul star, are somehow the souls of the dead Seraphim (potentially/likely split apart, a la horcruxes)? It'd explain why Ciro had to speak in rhyme to authenticate herself at the mercenary door, why Al has such a strong connection to the Last Seraph despite not even being FROM the Warrens. Finally, Rillborn seems reminiscent of what might be a seraph...
Note the giant fuck-off mouth reminiscent of Spira's monstrous form (FFS himself commented on this while confirming Theorycrafting 6 to be Winrar'd, so it's not just coincidence),
,the wings of an unknown, glowing substance (Seraphs in mythology are said to have wings of fire), and the fact that she's a worm, so she could be considered essentially a massive throat. And how does Cain describe the last Seraph?
wretched scream erupting from her thousand yard throat
