I edited my OP regarding what Alexander actually is, since I decided it's going to be left deliberately and permanently ambiguous by Frictional, and so probably isn't worth debating.
However, I think we can safely conclude that whatever Alexander is, he isn't merely a human who's achieved unnatural long-life via magic/alchemy, because one of the notes written by him towards the end of the game has him referring to humans as something foreign to himself. It's clearly implied in his manner of phrasing. I don't recall exactly, but he says it like "The humans..." or "These humans..." It was a choice of wording which referred to the species in a detached manner you wouldn't expect the speaker to choose if he were human himself.
It's a note in which he describes how humans have surprised him with the collective similarity in their response to torture -- he originally expected the responses to be highly individual -- and how it's ended up being more convenient for him in extracting vitae... something like that.
Also... one of the little white canisters
* you find in desks which white out the screen with a paragraph of text presumably authored by Alexander mentions how Alexander was originally banished from the dimension he's trying to return to via the Orb. It's also mentioned in a note by Agrippa (iirc) that Weyer had tried to negotiate Alexander being allowed to return to it.
So unless this other dimension Alexander was trying to return to is one that has indigenous human inhabitants, that's more hard evidence Alexander isn't human.
* What the heck were THOSE things, btw? The effect of reading them was unique enough that it seemed to suggest an other-worldly quality...
Quote:Some kind of reference to real things, requires a lot of time to wikipedia/google/dig it.
Are you just guessing here, or do you actually know what the meaning is?