She never forgot her cover id.

During her training at Langley, she was famous for it. Seventeen years old and she could keep more backstory in her head than agents years her senior. It didn't matter what her instructors did to confuse her (sleep deprivation, interrogating, her in the middle of dinner, interrupting her in the middle of her shower, intruding on her seduction lessons).

Sarah Walker always knew who she wasn't.