I'm still piecing this together, so take this with a grain of salt. Ratu Es isn't a character who gets introduced with a big fanfare; she's more of a creeping dread kind of presence. From what folks are saying in the 'Darkened Spire' forums, she's the ancient guardian of the Celestial Archives, basically the librarian of forbidden knowledge. Her role seems to shift depending on which protagonist stumbles into her domain—to the mage regressor, she's a potential mentor figure offering fragmented prophecies, but to the gamer-system guy trying to cheese the lore, she's more of a puzzle-box boss with reality-altering rules. There's a lot of debate about whether she's a neutral force or if her 'guidance' is actually a form of long-term manipulation. Her dialogue is always in cryptic triplets, which is cool but also a pain to interpret.
Some readers think she's the reason the timeline keeps fracturing in the later arcs. Honestly, her most interesting role might be as a mirror. She reflects the lead's true nature back at them—the strategist sees a labyrinth of logic, the brute-force warrior just sees a wall to smash. Makes you wonder what she'd show a true pacifist character, but we don't really get those in this genre.