Rosaline is a foil to Juliet. Rosaline is there to enable Shakespeare to contrast Romeo's conventional attraction to her with his real love for Juliet.
Paris is a foil to Romeo. On the other hand, Paris's conventional wooing of Juliet sets off Romeo's unconventional but undeniably sincere wooing. Paris acts like he is negotiating the purchase of Juliet, not trying to get her to love him.
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