I think it depend on how much affection point you have with the heroines.
You get affection points for the girls if:
You choose their option in ADV mode (+1)
They get the MVP while they're in Nozomu's party (+1)
They fight the Boss while in Nozomu's party (+1)
You choose a girl in the Izumo shrine sub-path (+2)
I just copied that info from a post in another forum. Still don't know if they're true.