The Sterile Cuckoo (1969)

Uptight college freshman Jerry Payne finds a carefree friend in zany Pookie. After an awkward meeting on the bus, Pookie quickly works her way into Jerry's life. She makes an unannounced visit to Jerry's campus, and before long annoyance turns to affection, and friendship turns to romance. But with Pookie's increasingly neurotic behavior, how long can this love affair last?