Don't look at me, that's the formal dress of a priestess according to the game =.=
Basically there are two ways of getting rid of the ghosts. Either you release them from their earthly problem, or violently purge them with rune papers. Not recommended, cause you will get a bad ending.
These Japanese games are full of futanari. Go google it up :P
You get rid of the girls by solving their quests, like find something that belongs to them or run a stage show.
Most of them you will not face any problem, except this one.
She will challenge you and wait for you at the courtyard. If you go there immediately, it will end in a fight and bad ending. You must exhaust all options possible to avoid the fight, like search all the rooms for stuffs, before talking to her.
Save the rune papers for the fight at the "eternal void"..