That one chick that Rachel claimed to make out with, and Phoebe didn't believe her. They went out to dinner with her and the girl said she didn't remember making out with Rachel. Then Rachel kissed her outside of the restaurant and the girl confessed her love for Rachel.
EDIT: Oh, her name is Melissa in the show, but that's irrelevant. WHO is the actress that played her?
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~Halo Give in to The Night.