It's kind of bland. I mean, seriously xD. The Key Person. Well, that summarizes her role very well >.>. Still, the "person" just kind of brings it down. Anything other than that, like "The Key Siren", or some other would've worked.
The magazine referred to her as a "key person", as in key to understanding the plot but at the same time a play on words because she uses the keyblade. It's not an official title guys. :/