She's something; sex fairy, sex genie in a crystal, whatever...
She's waiting for him to croak either naturally or unnaturally when it all comes down to it
Treasure of Nadia is flaky and obviously rushed these days as evidenced by all the bug fixes. And it never seems to matter to anybody what happens in the game, what things are called in the game, or what things are in the game. (Apart from me.) For example: After the MC, Diana, Clare, Janet, Alia and Kaley jumped into the whirlpool in the Spanish Fort and got swept by the current into Sofia's mansion immediately after leaving the water they were all completely dry! WTF? And how could Henry and Evie be flesh and blood and getting it on in limbo while Henry's dead earthly body was being disposed of by Duncan and the Eclipse Guild? Even in fantasy games there should be some adherence to the game's internal logic, natural law and physics unless the game has altered versions of these things and announces so explicitly. (Like a game taking place close to the speed of light or near a black hole for example.) As for Evie, well, unless Henry is going to do a Sofia and make himself immortal by quaffing water from the Fountain of Youth, or by some other means, he 100% will die sooner or later so she won't be disappointed, but since neither she nor he would be physical beings after death they wouldn't need, and so wouldn't be gifted with, sex organs and so couldn't bang like bunnies or even bang at all!
A few things the Evie character said hinted that she herself was on her way "downwards", shall we say, maybe to the demon world, possibly after bargaining for some kind of deal with some power, someone or something to restore Henry's life. Trouble is if Henry does help to save the world, his destiny according to Evie, unless he goes over to the darkside, or is a very naughty boy afterwards, he definitely will be on his way "upwards" after death and so unlikely ever to see Evie again unless the two meet up in limbo and become flesh again, which is possible according to the gospel of NLT Media but impossible according to historical, magical, traditional and occult authorities as far as I know.
I am not trying to attack NLT Media or Treasure of Nadia, gratuitously, because I consider that the games they produce are always of a high standard, but to point out that to reach the highest standard possible The Genesis Order needs: (1) To be internally consistent and make better sense in its own terms to players; (2) To have an equal amount of effort put into its story as its consistently brilliant art. A game like that would shock the world - in a magical, weird and wonderful way - and be something that legions would pay to have the privilege to play I am absolutely sure!
N.B. The nearest thing I can think of in mythology that the Evie character resembles is a Djinn, or Genie, like the one in Aladdin's lamp! That's just my opinion of course and because NLT Media haven't actually revealed what she is, or ever likely to now that the game is ending, we'll probably never know for sure and all doomed to harbour different opinions, theories and ideas to explain her which might be what NLT Media wanted to happen all along. Or maybe we'll discover what she is, how she became trapped in a crystal, and where she came from in ToN's bonus content. Now there's an idea! What about it NLT Media!? Why not make Treasure of Nadia's bonus content a full-blown biography of Evie, former prisoner of the Soul Crystal?