@Indigo Was it you who I quoted with I have a darker theory for Kuina?
If not, to make it as short as I can.
I think Koshiro being from old gen Wano he might've been a strict mofo and always wanted a son but got Kuina. The "Stairs" bullshit might be actually Koshiro offing Kuina in some sort of twisted love cause he heard all the shit Kuina cried about when he told her she will never become strong as a swordswoman.