The question of who "E" is might prove to be one of the most important ones we can ask over the coming weeks.
I'm viewing it as somewhat of a Chekov's Gun* at the moment - the only interaction with "E" that we've had is that one mail around Dragon Bash which asked us to contact Marjory Delaqua (and tell her "E" sent you - she later confirmed she has no idea who it is).
Whilst this brief interaction might not suggest a large role at the moment, plot devices such as a "mysterious stranger" are rarely inconsequential when applied in good storytelling. So, as our various questions get answered and we begin ticking off the remaining mysteries, the ones left unanswered will become more and more important (and suggest a centrality in the narrative).
Contacting Marjory has proven to be a pretty good move - she has been instrumental in taking down the Tower of Nightmares and puzzling out Scarlet's clues in the Dead End. This would seem to be a pretty big tick on the "Good" side of the debate regarding E's intentions.
On the other hand, who knows how deep the plan might go? Perhaps E's Machiavellian scheme includes bringing Marjory into the equation. Without Jory and Kasmeer's anti-toxin, Scarlet wouldn't have been able to perfect her poison, after all.
It's difficult to say at this stage. I've got a feeling one of the continuing threads after we finally take Scarlet down will be either the identity of the big-bad who is controlling her or the identity of E (or both!); and it will be this question which will take us into the next story arc.
*"If you say in the first chapter that there is a rifle hanging on the wall, in the second or third chapter it absolutely must go off. If it's not going to be fired, it shouldn't be hanging there." - Anton Chekov, 1911
ps. Could it be Ellen Kiel? ANet wouldn't call them "E" without "E" being in their name. Plus, the story would work just as well if we'd voted Evon Gnashblade into office (?) I dunno. Just wankin' in the dark here.