I could see a few possibilities. The most straightforward would probably be childhood friends where they've had each other's backs for a long time before Shisui gets swept up in royal court stuff so she asks Hiroe to accompany her. Could also do something where Hiroe is just there as a suitable job for a lady of good breeding but she protects Shisui from some (possibly supernatural?) threat(s), Shisui figures out that Hiroe = Zenko but protects her secret, and that grows trust and respect between them. Obviously the first of those works better for Shisui not knowing her secret yet but having that trust between them already.

I liked s1 a lot, but as far as I can tell that's all Netflix has. My first choice would be to watch s2 first, but since I haven't managed to find it yet and not sure when I will, I'm not really against some spoilers!