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.