A Key to Exile is a person who possesses the genetic marker allowing them to control an Exile.
Only females can be Keys, and each Exile can have only one active Key at a time. When a Key to Exile dies, their closest relative who carries the genetic marker becomes the new Key.
Speaking a Mysteria to a Key will result in a powerful reaction as the Key goes into a trance-like state and releases a pulse of energy before speaking the second part of the Mysterion. This pulse of energy has been shown to affect the weather, raise the pressure of Claudia fluid, shatter glass, and destroy incoming projectiles. The speaking of even one Mysteria will affect an Exile, though all four are necessary to fully control it. An Exile's defense mechanisms will also refrain from destroying a ship carrying a Key.
Because a Key can be used to control its respective Exile by anyone who knows the proper Mysteria, regardless of the Key's will, they are highly sought after by those who seek to harness the power of Exile. Emperor, Maestro Delphine Eraclea, Alexander Row, and Luscinia Hāfez are all characters who have sought to obtain a Key to Exile for their own purposes.