We´re obviously led to think it´s Mike. Imho even too much so, that it convinced me it´s not gonna be him.
That´d be so obvious and telegraphed at this point, it wouldn´t make any sense to drag it out for this long, as some sort of big reveal.
Still think it´s Mark (my words...

) Some of his background and timeline match, there´s hints in the code + this one possible nugget in Jeremy´s dialogue, where he says he saw Gillian party with "some asian girl". (Mark´s sister?) Could as well be nothing but then again, why would he need to mention the girl being asian? If so, then who is Mike´s girlfriend? We know she came to town because of a job change recently, right? Other than Gillian this only matches who so far? Nat.... or Eva´s just trolling us and Gillian´s bf is just some complete rando we´ll never see