Kaguya

This princess appears in the oldest known Japanese folk tale. The tale ends with her heading to the moon with a heavenly entourage. Some say she may have been an alien all along.

How to acquire?
It is by completing all six Kaguya sealstones to unseal Kaguya. Sealstones are aquire from stories of Chapter 3 or stealing from other ghost agents by battling them. Completing again the sealstones will acquire Kaguya Magatama.