Otherwise it seems like she's just a plot device for Rowan and Alexia to use in order to advance their own stories, which is... Disappointing. Shaya's kind of already fallen victim to mostly being a Jezera plot cog despite getting a few scenes and even CGs dedicated just to her, and it'd be a shame if X'zaratl follows suit. Besides that, Nasim already seems to fill the role of functional plot cog in terms of the fertility treatment storyline.
An apt observation.
One thing to consider, is that the game's overarching story arc is
long. Really long. And many of the game's characters will accompany us through all of it, and ideally should have content for most of it This is the primary reasons why we're not exactly learning much about some of them. Their personal backstories will be explored later, mostly in Act 2. This goes primarily for Cliohna, who's exact goals are still unknown to the player as of now. For X'zaratl the player will be able to get a little peek behind the curtain, though her storyline is... Less straightforward that most. Since Alexia and Rowan are such a big part of it, and these two can both pursue their own corruption plotlines, she has to react more strongly to changes in their dynamic than any other character in the castle.
For example, there is an entire different route for her latest event, that has yet to be coded in (I have to discuss the details with Winter before it goes live), that triggers if Alexia is on Jezera route - and it's also the reason why the event is called JezeraNTRdetox in the game files.
The second thing to consider, is that the game has a limited number of weeks to work with. With 70 weeks to work with, a 7 storyline - which I think Shaya has at the moment - takes 10% of the game's weeks. We simply don't have enough time to give every character their own long storyline. This is the reason why Nasim's storyline, which was orignally planned to trigger mid-way through act 1, was pushed entirely to act 2.
And if he feels like a plot device for the fertility treatment... It's because he is

We wanted to give the player more opportunities to corrupt Alexia without her cheating on Rowan, so he got roped into the plotline against his will.
As for virility treatment, that was a random idea I thought would be hot.
Sometime's it's that simple.