She can be anyone, except herself.

Echo is a member of a highly illegal and underground group of individuals whose personalities have been wiped clean so they can be imprinted with any number of new personas. Confined between missions to a secret facility known as the Dollhouse, the "Actives" are hired by the wealthy, powerful and connected to wholly become—with mind, personality and physiology—whomever the client wants or needs them to be. They know no other life than the specific engagements they are in at that time—or do they?