Benvolio is considered to be more of a dramatic foil to Mercutio. A dramatic foil is a character that brings out the exact opposite character traits of a second character, thereby emphasizing those traits. Benvolio is Mercutio's dramatic foil because while Mercutio is extremely hotheaded, impetuous, and rash, Benvolio is one of the most calm, rational characters of the play; he is even one of the play's peacekeepers. However, while they are for the most part opposite characters, they do share the similarity of both being genuinely friends with and caring for Romeo.
We especially see their mutual friendship with Romeo in the fourth scene in Act 1 in which they try to persuade Romeo to join them in crashing the ball. While Mercutio acts more playfully, treating Romeo's heartache as a joke and Benvolio is more serious, both seem genuinely concerned about their friend's emotional state and want to see him set aside his feelings of pining for Rosaline. We especially see Mercutio's concern for Romeo's well-being when Romeo says that he is feeling too heavy to go dancing at a party and Mercutio replies, "Nay, gentle Romeo, we must have you dance" (I.iv.14). We also see Mercutio taking Romeo's feelings more lightly than Benvolio, treating them as a joke, when he tells Romeo to be rough with love and to "prick love for pricking," both of which statements can be viewed as sexual innuendos (28-29). In contrast, Benvolio acts more seriously in this scene, each line encouraging Romeo to give in and join them at the Capulets'. Prior to this scene, we see Benvolio genuinely acting concerned for Romeo's welfare, even begging him to forget about Rosaline. In addition, crashing the ball along with Mercutio, who was invited as relation of Capulet's, was Benvolio's idea as he thought it would give Romeo a chance to compare Rosaline to other beautiful women, thus distracting him from her.
Hence, while Mercutio and Benvolio are generally considered opposites, they do share friendship with Romeo in common, and genuine concern for his emotional state.