Maybe Muramasa’s been wandering around for a long time without Kouga, sick and obsessed with freeing his wielder. He stumbles on a ten-year-old Ichigo when he’s wandering around the Human world one day, and he can sense the ridiculous amount of reiatsu Ichigo has, and it’s interesting enough for him to stick around for a little while. It isn’t as if Karakura has a shortage on Hollows either, and he’s careful to stay out of sight; someone with that much power would be able to see him if he showed himself.
He watches Ichigo buy groceries and drag them home, watches two little girls even younger than the boy latch onto him at the door, watches him help them with homework and walk them to and from school and tuck them into bed. Muramasa isn’t exactly an expert on children but this strikes him as a little odd – shouldn’t there be parents around to take care of them?
And then Muramasa finally sees the father, and he only needs a few moments to place that face. Shiba Isshin should not be playing house in the human world; as a Shinigami, he should be in Soul Society.