The half Cat-Sidhe girl and the Illuminati Spy

This is a Blue Exorcist book, I've had this idea bubbling in my head for a while now and I'm just now starting it.

Emi looks like a normal girl, or that's what others believe... A gold necklace rests around her neck to protect something but what?... She was captured by the Vatican, for what? How does she know Shima? What was her kind known for in Kyoto?

Follow the tale of how a Pink haired illuminati spy working to become an Exorcist, see's this strange girl he knew as a kid... and possibly find love in her despite not wanting to?.

And what group is she apart of...that no one knows of? Why is she so.... Indifferent?