That she used to be a member of Team Flare, specifically Lysandre's right hand. Most people don't know much about her 20s, and I imagine Briar lies about her life before moving to Galar so people don't suspect her, she's specially afraid of having agents of the Interpol spying on her so she just says she went back to Unova right after finishing college (but this is a lie, since she stayed in Kalos to join Team Flare)