Yep... Kaguya-hime. Yep yep... Oh this is gonna be good. I remember seeing her reveal in the four-part anime finale. Such good teases.
We still need to have it spelled out that Tsukasa isn't Kaguya from the tale, but someone that grew up around when Kaguya was in the Emperor's court, most likely. As far as I can figure, the only real connection between Tsukasa and the classic tale is this, as well as Tsukasa ending up getting the immortality elixir mentioned at the end of the tale instead of it being burned up on top of Mt. Fuji, to explain why she's so long-lived and seems to be frozen at the age of 16.