Dr. Stevens (TV Series)

The Unnamed Woodbury Doctor can be seen behind Michonne in the third season official trailer. She is a resident of Woodbury.

Pre-Apocalypse
Nothing is known about this unnamed Woodbury doctor's life before or as the apocalypse began. It is possible that she was a resident of Woodbury before the apocalypse began. It is also possible she was a trained doctor.

Season 3
This woman is currently acting as a doctor within Woodbury, giving assistance to sick citizens. Whether there are more active doctors in the town remains to be seen.

Trivia

 * Excluding the extras in TS-19's opening flashback, she is the second known doctor in the Walking Dead universe, the first being Hershel Greene.
 * She may not be a doctor, but a nurse acting as a doctor in the town. If there are doctors in the town, it would be safe to assume she is an assitant rather than an acting physician.