Izutsumi was given to Maizuru as a pet by Shuro’s father, though in practice she was just another slave purchased to become a retainer. And Shuro’s dad’s actions are questionable af as he was love-bombing Maizuru by giving her a literal person as a gift.
Unlike with most retainers, Izutsumi was never happy to be owned just because the Nakamotos housed and fed her. Likely because typically retainers start off very young so they can be indoctrinated into endless gratitude towards the family, ready to die for a Nakamoto at the drop of a hat.
But Izu was purchased when much older, as a drunken impulse buy to “get Maizuru a kitten”.
I’ll post Maizuru’s world guide chapter next, as it touches on these things.