The Longest Journey/WMG

The Kin are Sufficiently Advanced Aliens
They came to Earth to secretly assist its evolution like the Progressors from the Strugatsky Brothers' books.

The Kin are what is known in Stark as "Archangels"

 * Red = Michael
 * Green = Gabriel
 * White = Raphael
 * Blue = Uriel

All Kin are Shifters
Cortez referred to Vestrum Tobias as if knowing him personally, whereas Tobias is obviously non-Shifter and looks like he had a reasonably human lifespan. It would make perfect sense for the protectors of the Twin Worlds to be able to cross the Divide.

If that is true, it may also be that all Shifters are descended from the Kin.

Lady Alvane is the future self of April
It makes sense because she is pals with Crow and lives between the worlds, which is suggested to be a timeless space (similar to the Venar worldview).

Lady Alvane is related to Kian Alvane
Duh, same family name. She is either mother/grandmother/ancestor or the Future!April who married him.

Lady Alvane is the White Kin
She would want to help her daughter/sister and she knew Crow.

Lady Alvane is Future!Zoe
She is the only other woman whose sidekick Crow was.

Shifters are inherently disruptive to the Balance
Because they create unregulated traffic between the Twin Worlds. That's why they are so rare: the Twin Worlds kill them off.

Equal portions of tech and magic slipping across the Divide would not disrupt the Balance
This is one way to prepare reunification of the Twin Worlds.

April is in love with Cortez.
After a conversation with spirits in Banda Spirit Dig, an entry appears in her diary which says, among other things, this:

"As for Charlie...does he really LOVE me? I mean, like, love-love me? I had no idea. No, that's a lie. I guess I had some idea, but I didn't really want to deal with it, because I just don't feel that way about him. He's sweet and kind and...I simply have a tendency to fall for the wrong guys, for the wrong reasons. Like now. Like with...oh, no, I'm not even gonna think about THAT."

So who's that wrong guy? Given that she says "now", looks like she knows him ("guy" must mean "male", right?) for a short time and, therefore, the guy must be featured in the game. Of all the male characters, she has the closest relationships with Crow and Cortez. April and Cortez appear to have some spiritual connection (she has dreams that show what happens to him). Note how bad she feels about his death: even in Dreamfall, 10 years after his demise, she still can't bring herself to say that he's dead: she refers to him as "gone". Sure, he was her only guardian and mentor, but isn't it too much of a reaction? Besides, Emma teases April saying that she (April) has romantic feelings for Cortez. Emma is an expert in such things, she would have noted something April herself refuses to acknowledge. And, after all, given that April and Cortez belong to the same species, it's only natural for her to be attracted to him. Also, if it's Cortez, April's disapproval of her feelings is understandable, because not only he looks much older than her (and is even older), but everybody knows him as a crazy old man.
 * April is attracted to Cortez because he gave her a sense of purpose. As Dreamfall shows, she isn't really capable of making sound judgement about her goals in life without direction from someone older and wiser. Does that mean she loves him? I don't know but she certainly depended on him a lot--not in any immediate practical way, of course, but as an inspiration, as someone who tells her she is needed.