From The TV IV
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David Sinclair is a young FBI agent originally from the Bronx, NY. By season three, Agent Sinclair has worked under Eppes for longer than the other agents.
In the pilot Don was skeptical of Sinclair, because he thought Sinclair was put on his team to spy on him to the Director who questioned using math to solve crime. However, over time Sinclair has proved his worth to Eppes.