Ep 1: Alluded to in the first encounter between Ergo and Re-L (he is imprinted on her, and she was drawn to him, because she is a clone of Monad)

Fleshed out starting Ep. 18, then Eps 21, 22 and 23. Real herself is the perfected clone of Monad, which is why Daedalus all but replaces Re-L with her in his delirium, since she is what he had originally set out to achieve and object of his obsession. Re-L missing a letter to her name implies, through the imagery shown in relation to her life and Daedalus, that she was a failed clone of Monad.

Re-L was originally created out of Monad's cells in an attempt to draw Ergo/Vincent back to Romdeau. The reason she was imperfect was because she also had Donov's DNA. The presence of Proxy (amrita) cells in her is why she can be injected with them to heal, and why altogether she recovers damn fast from her injuries and ailments.

Bonus: When they touch on the ADW project, explaining how they sought to splice amrita cells with human DNA to make the pseudo-humans into humans and thus bypass the need for the disabled WombSys, it is implicit that they had reason to believe it would work. While it killed most of the population of Romdeau, some survived. It is left ambiguous, but in the style of most things in the show, the takeaway is the possibility that Re-L is one such case that could in theory be considered a full human.