After S4 E1

Hotch felt the need to hire someone with a medical background to the team.

You were the perfect person for the job, a paramedic who took profiling classes for fun. After going through the FBI Academy, you went with Strauss to meet your new team.

You walked in as Strauss left and Hotch went to shake your hand.

Hotch: "You're the new agent, I assume. I'm Agent Hotchner. This is the BAU team, Agents Rossi, Morgan, Jareau, Prentiss, Garcia and Dr. Reid"