Denise was Phoebe's roommate in Season 6. When Rachel needs a new place to live after Chandler moves in with Monica, Phoebe first avoids having Rachel move in with her by saying that she already has a roommate, Denise. When Rachel can't find another place, she makes her mind up and says that Denise has moved out and won't be back until the 26th of December. Rachel jokingly says that Denise may be Santa, to which Phoebe shows a puzzled face as she thinks this may be true. Since in "The One in Vegas, Part 1" Phoebe says she lives alone, it is possible that Phoebe made Denise up.