Princess Nastoya is, surprisingly, an Elephant in disguise. We have to ask why she got to go undercover as a Princess while Elphaba had to go undercover as a Witch – seems like a crap deal there. (Though Witches arguably have cooler hats.)
Aside from being a not-so-pretty princess and an Animal (in an Oz where they are increasingly rare), Princess Nastoya is also the novel's counterpart to Madame Morrible. This book really does love mirroring characters. Princess Nastoya only has one major scene in the novel, but it's a doozy.
She does an all-out Yoda routine with Elphaba and gives her nice and cryptic advice. Nastoya arguably does more than anyone else to push Elphaba toward becoming a Witch, and it's notable that Elphaba's sorcery ability only really starts to manifest itself after she starts "posing" as a Witch. Perhaps Nastoya knew how to push Elphaba toward her destiny. Whether or not that was a good or bad thing is debatable.
But the issue of destiny is really interesting here. Princess Nastoya encourages Elphaba not to be at the whim of destiny:
"Listen to me, sister," she said. "Remember this: Nothing is written in the stars." (188.8.131.52)
But the Princess also seems to be orchestrating Elphaba's future. She basically arranges for Elphaba to become a Witch and prods her into actually growing into that assumed identity. The cryptic advice that Nastoya gives Elphaba also seems connected to Elphaba's ultimate fate (the one that the readers know). Is Nastoya trying to subtly tell Elphaba to fight against her doomed fate, or is she cleverly arranging matters so that Elphaba's fate will play out as we are expecting it to: with a rather inglorious death via water bucket?
As with other characters who meddle in Elphaba's fate (Yackle, Morrible, the Wizard), we are never entirely sure what Nastoya's motives and meanings are. Elphaba sees Nastoya as a kind soul, though, and we are inclined to agree. She does show Elphaba a lot of compassion. And Nastoya's role in Elphaba's life it also alluded to earlier in the novel. When Fiyero enters Elphaba's apartment in the Emerald City, he notices this:
Most effectively, and gruesomely, the skull of an elephant hung on a rafter, and a bouquet of dried creamy pink roses emerged from the central hole in the hull of its cranium – like the exploding brains of a dying animal, he couldn't help thinking, remembering Elphaba's youthful concerns. Or maybe an homage to the putative magical talents of elephants? (3.1.40)
Elphaba really does seem to be surrounded by portents, symbols and themes her entire life, Nastoya being one of them.