I have to say I don't have any good candidates. My two guesses are a Jakobite instance or Erin's dark alter ego, but those are long shots.
Beyond the fact that the answer to all questions is Rain,
all the answers I can come up with to this question feel like long shots.
One approach: who else has died or disappeared, or is a magical figure? Zenon? Polly (if she isn't Rain)? John Wilmot? Rain's father (Atlas?)? Mallory's old friend Charlotte? George Mallory (climbing Atlas)? The blood monster? Might Julian's recent skirmish have left part of him in there? Or, well, we haven't seen Derek in a while...
Another angle is: who would direct Erin towards Adrian and stop Adrian whacking Vik? That seems more Rain than Jakob.
Was Kanti definitely one of the women who emerged from the motel? Or, if Erin has a dark alter ego, maybe Cherry has a light one?
There was also that guy in the room next-door to Vik in 817. Maybe he's a Wizard of Oz figure around here.
By the by, Aiken seems to have adopted the mannerisms of a genie in this page. Maybe Aiken's fakin' and he's really the one controlling The Cloud.
There. I've named most of the cast. I'm fairly confident this will prove to have been a process of elimination.
