Stage 4 boss, the Phantom Ensemble. Noisiness times 3. They mainly have the ability to handle musical instruments and give musical performances without using their hands and feet—times 3.
The Saigyouji family enlisted the Prismriver sisters to provide music at a convention, promising them a flower-viewing in return. The three of them are always summoned to banquets and such; their music stirs things up. They're like ghostly versions of publicity-generating cultural icons.
This time, the Saigyouji family is holding an extravagant flower-viewing festival, so they would naturally be invited.
The three of them were born long ago. At that time, there was a human noble, Count Prismriver. The Count had four daughters, whom he cherished.
However, due to an unfortunate accident, the four daughters were orphaned. Without any other relatives, each daughter went her own separate way. The fourth daughter, Layla, was unable to leave the mansion that held so many of her memories. Using her strongest power, she conjured poltergeists with the appearance of her older sisters, then vanished along with the mansion and the poltergeists.
Time moved on, and the death of the four people from those old days came to pass; the mansion, now called the Poltergeist Mansion, remained in Gensokyo. The three poltergeist sisters continue their noisy lives in that mansion to this day.
The musical performances given by those three, at various places of announcement, a fusion of casual rhythm and classical instruments, are loved by the inhabitants of the cheerful Netherworld. Yet, the performance itself provides no additional effect, and does not heal.