After some thinking, here's what I know about the new boss:
1. She's either a spirit, or involved with spirit (since both Reimu and Miko, the two who are known to be able to listen to spirit, have a clue about her).
2. She's invisible to the naked eyes, or otherwise have such a low presence to leave no physical tract (Since (presumingly) no other ULiL, not even Hijiri or Reisen, know about such indivitual other that through the Occult Ball).
3. She's smart enough to used the Occult Ball, since the kanji 憑 (tsuki; possession) is pun with the kanji for the moon, thus the connection.
4. Possession is her main theme and expertive (this is kinda obvious).
Unfortunately, that still leave many suspect. But perhap this would help in our speculation.