Edison Rooker — Rook — seems to be your normal average teenager looking for a job until the real reason why he singled out Antonia's shop is revealed. He doesn't have any magic, and the closest he's ever been to the magical community was when his grandmother was alive. After his grandmother's passing, he created the Spell Binder, a device for detecting magic, to be able to regain access to the magical world. The problem: technically, the device is illegal; and the fact he has it, a non-magical person, makes it worse.
On the other hand, Sun is magical through and through. We meet them as Fable's (Antonia's rival) apprentice on their way to deliver a box of cursed items to Antonia's shop to be "fixed". If Rook needs the help of the Spell Binder to see the ley lines, Sun's special ability allows him to see magic (in addition to being able to use / wield it).