Dr. Schacter is a neurosurgeon who worked on Derek's hand after the plane crash.
Following the severe injuries to his hand in the plane crash, Derek consulted with Dr. Schacter and Callie Torres to figure out a plan to restore more of his hand function. He conservatively estimated that he could get Derek 80% of his hand function back. However, Derek wasn't satisfied with this plan and when Callie proposed an alternate solution that would get him back to full function, he convinced her to try that instead and Dr. Schacter's plan was abandoned.
Dr. Schacter is an attending neurosurgeon at Seattle Grace Mercy West Hospital. According to Callie, he's a great neurosurgeon, one of the best.