Thanks for the close-up, Arcorann. At least we can see something.
And I hope this new boss is not Prismrivers Sister. Though I am guessing this (new) boss uses some sort of stringed instrument, considering how it looks like she is dropping notes out of strings (or maybe its a music sheet, you decide)
EDIT: Actually, I think the boss int he screenshot is none of Prismrivers Sisters. The reason is because when you face Sekibanki or Kagerou as mid-bosses, you don't see their names in upper-left corner. If you were facing against Cirno, however, her name is still shown. So its safe to assume its a new boss.