Impulsive, emotional romeo acts as a character foil for
rosaline, who is unloving and cold.
capulet, who is stern and protective.
nurse, who is wise yet funny.

First of all, you need to know what a foil is. In literature, the term foil refers to a character who contrasts with other characters - usually, their qualities are quite the opposite from those of other characters'. Having that in mind, Romeo acts as a foil for Rosaline, who is unloving and cold, whereas Romeo is impulsive and emotional.

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