Laurie was one of several children's ghosts that were imprisoned by the Soul Catcher and bullied by Benny. She managed to escape and appeared in Peter Venkman's dream to tell him about what was going on. The Soul Catcher tried to get her, but was prevented from doing so. When the Ghostbusters finally found, defeated, and trapped the Soul Catcher, Laurie and the other children's souls were able to go free and enter the afterlife.
Egon verified that Laurie is a full floating-repeating phantasm.