The telepathic link between Siera and Spirry formed upon the creation of their friendship. At first, Spirry offered it to Alicia (I think; it's one of Inferi's character, I think), but she replied no thanks. So, Siera offered to help Spirry, to save him, and so it is, a friendship was established. As Fishing pointed out, after the appearance of the Playroom, since Siera entered it as well, this formed a connection between the two. Both entered the Playroom in an ethereal form, and so formed a mental connection.
Spirry asked what she truly wanted, and she answered, now he is more interested in her. This was overall indirect, mostly alluding to my own mythos, but it also means Siera can influence Spirry. It was meant to be a backfire effect: Spirry can mind-control people in my works, but he can't do that in Engelain, and now, Siera can actually mind-control Spirry.
Of course, it's possible I did not adequately express this theme, or may have misremembered a few things, or misunderstood the plotline, or gotten mixed up between Spirry's mythos and the storyline of Engelain. I will wait to see what Angel has to say, as this would clarify everything.
08-Feb-2018 04:33:22