I have the suspicion that those suit wearing ghosts were people just like Mieruko's father, and that Mieruko's father is probably going to end up becoming one.
we got a comparative lore dump on how the ghosts work, though I do prefer it being drip fed into this sort of ambiguous thing that definitely has a logic to it.
i hope we find out whatever caused Miko to have this curse if there is anything more to it other than she was born with it, i really do hope she can be free of it by the end of the series
My guess is that she was just born with it,like most psychics are,like her dad probably was too. After he died,maybe his power passed on to her and something turned them on all of a sudden.