Physic is the physician of the Black Swan. She wears a mask to conceal who she is, and Physic is not her real name.
Physic was the person who helped Fitz after he was impaled in Exile (Neverseen). It was said that Physic was in the collective until she couldn't always be there, so she gave her place to Juline Dizznee, as she was her doctor. It is said that the reason she gave up on being the Collective is so that she could focus more on medicine. This is because when Sophie had an accident in the Forbidden Cities with limbium, something she is allergic to. The story doesn't go into detail about the topic, but it had something to do with Mr. Forkle and Physic.
Physic is shown to wear a fancy, pink and red Mardi-Gras type mask to hide her identity.