Right now she’s at odds with whatever is in her head. But it seems to have been passed down - she got it from her grandfather. My really wild guess is that it was trapped by The People somehow, but before that it was the same being as the one living in Myrddin’s shadow. Myrddin seems to have made a deal with it. What is the nature of that deal?
Well, Thaddeus’ first guess back when Sebastian suggested it was that dual casting would require some kind of mental damage. Siobhan wondered if the thing in her head was part of why she could dual-cast. and Myrddin’s journal was specifically written to only be readable by someone who can dual-cast. In other words, by someone who has the same Aberrant that Myrddin did. So my guess is that the journals were very specifically intended to fulfil an agreement with that Aberrant. An agreement that, for both better AND worse, Siobhan is going to be pulled into.
Who knows what comes with that?
As for other ideas for growth, I’m VERY interested in what will come of Siobhan’s symbol. I started a public thread about it at What will Siobhan's symbol be able to do?. That explains why I think it is going to be so fascinating to watch. I know that @AzaleaEllis loves creating interesting magic systems. This symbol so clearly ties to everything creative about the magic system that finding interesting uses for it pretty much has to draw her like a cat to catnip. I really look forward to finding out what she’s come up with.