Thoriane's patrons are chosen from a pool of those who have made contracts with her. Typically her Patrons have more open-ended or long-running contracts, and have jobs or hobbies that Thoriane finds useful in some way. Thoriane may not directly tell her Patrons they have been chosen, but she will appear to them repeatedly - in dreams or in person. The Patron status is also clear to other Guardians. There is no visible mark to indicate it to the average person.
Mika - as Bounty Hunter on left, Korthan outfit on right
Rose
They generally have the ability to interact with shadows - using what they typically work with in their day to day life to create black portals. While these can be travelled through, it is rare for a Patron to know how to safely teleport through shadows. They are far more commonly used to allow Thoriane easy access to a location.