Lucy, played by Kimmy Robertson, is a high-voiced, childlike blonde receptionist at the Twin Peaks' sheriff's department. She is seeing deputy sheriff Andy Brennan, until they have an argument. She reveals later that she was also seeing Dick Tremayne, who works at Horne's Department Store, after feeling frustrated with Andy. Lucy gets pregnant, which turns out to be the reason why she and Andy have been fighting. The paternity of the child is called into question when Andy reveals that he cannot be the father of the baby, due to low sperm count. He gets retested and finds out that he could have been the father. This sparks off a competition between Andy and Dick Tremayne, both vying for Lucy's affection and the role of father to her child. This includes adopting "Little Nicky" through the Big Brothers program. After some particularly rude behaviour from Dick Tremayne, Lucy decides not to wait for the baby to be born in order to determine paternity and chooses Andy as the father of her child.

In 1990, Simon & Schuster Audio released a cassette entitled "Diane ...": The Twin Peaks Tapes of Agent Cooper which compiled many of the recorded diary entries of Cooper that had been featured in the first season and the beginning of the second, along with specially-recorded entries including several taking place prior to the pilot episode. One of the specially-recorded entries takes place the night of Cooper's arrival in Twin Peaks and has Lucy giving Cooper directions to the Great Northern Hotel.