Mia was a bartender at the bar Osiris hits to pick his victims.
She is working when Dean comes in and asks for a drink. He asks her about the other men, but Mia tells Dean she wasn't working last week. The two begin to flirt with each other, and plan to get together after Mia finishes work. When she finishes work, she waits for Dean, but only finds his cell phone. When Sam calls for the third time, she picks up and reveals Dean isn't there. So Sam comes in person, and gets the phone from her.