It's possible there could be some connection between these two from childhood, and I could work with that into some CR.

Rei (and her cousin Eiji) are both nobles from the wealthy, influential political Hino dynasty. Rei's the heiress, though due to her father basically being a terrible human being she moved in with her estranged grandfather to manage a small shrine. This happened when she was seven or eight, and before that her family would've paraded her around and propped her up. She's kept a lot of her noble habits and manners, but for the most part lives a more quiet (and much less glamorous life) as a priest.