Seen at AD2225:

At the beginning of every episode we meet a new personality, the main character, Shiratori, does a children’s book sort of telling of meeting another princess, that all live in this castle. I think the castle is like Kozue’s body. (The landlady.)

The word "every" tells us that he's very early into the story. There aren't that many of them, it's not a Sister Princess.

I am pretty sure that Shiratori did not think of the castle as the body, if only because the transformations include the bodies. It's only a castle. BTW, at some point the prince enters the castle... Too much Freud is bad for you.

And I’m wondering how they met in the past fits together. Every now and then I catch Kozue saying something suspect that feels like it’s leading up to something.

I find it difficult not to throw heavy spoilers around while talking about Mahoraba at the level of detail AD2225 aims at. Perhaps it is a good idea to maintain some kind of out-of-line page. This is what Wordpress has "pages" for, after all... Maybe later.

I think any idiot understands after one episode that Kozue and Shiratori have to get together. Sakura petals were falling when they met, so... It's the law of the genre. It wasn't their first meeting, but since no Childhood Marriage Promise was involved, this one is presented as fateful. Since it's unavoidable, a small spoiler: circumstances jogged Shiratori memory later and he remembered seeing Kozue before, but it did not play a huge role, just a nice play with the concept.