From Deadline Hollywood:
Rachel Nichols (Star Trek) has been tapped for an arc on Criminal Minds later this season. Criminal Minds had been planning to add a new female regular cast member in the wake of the A.J. Cook's departure. I hear the character Alias alumna Nichols will play, FBI cadet Ashley Seager, was originally envisioned as a new regular but that has changed, and Nichols will now appear in 3 episodes. Seager is invited by Agent Hotch (Thomas Gibson) to consult on a case after learning her family history will bring a unique insight into the mind of a serial killer.