Hm. I don't know if there's an instruction manual somewhere, but I can tell you the basics for now.

[ He pauses in case Lea wants to interrupt and say 'nope' but if that doesn't happen, he continues on. ]

A Persona is a part of your mind or soul that manifests to protect you when you are in danger. You can learn to call it forth at any time and use its powers. Or you can call it forth using a device called an Evoker.