She's the nurse who treats Dan at the hospital for his mononucleosis. Her role is to tempt Dan into breaking his no-sex discipline. When they first meet at the hospital, Dan flirts with her—perhaps problematically, as we discuss in our Steaminess section—and even Socrates sighs at her attractiveness (4.102-113).
Socrates later forbids Dan from carnal pleasures, for the sake of his training, but he sleeps with Valerie nonetheless (4.232). He then swears to Socrates he won't do it again, and has to face a somewhat humiliating night with Valerie where she sends him home due to his unwillingness to break his discipline. She even goes out with one of his teammates the next week. Ouch.