When he appears in the novel, Rinaldi functions as a literary foil, his characteristics contrasting and emphasizing Frederic's identity and personal traits. This is seen clearly in Chapter XXV when Frederic returns after having been wounded in battle. Rinaldi is in serious decline, the result of too much war and too much work. As a surgeon, during Frederic's absence Rinaldi has endured almost continuous physical and emotional stress in putting too many bodies back together. "This war is killing me," Rinaldi tells Frederic.
As Chapter XXV develops, the contrasts between Rinaldi and Frederic become clear. Rinaldi drinks a great deal and suggests he and Frederic get drunk together; recovering from jaundice, Frederic has sworn off drinking. Frederic tells his friend that he will have a drink with him, but he will not get drunk; he notices how much cognac Rinaldi pours for him, saying it is too much. Rinaldi's excessive drinking emphasizes Frederic's responsible abstinence. It also suggests that Frederic will not attempt to escape his own painful experiences by losing himself in alcohol. Rinaldi's weakness emphasizes Frederic's strength.
Rinaldi serves as Frederic's foil in another very significant way--his attitude toward women. When he attempts to discuss Catherine in sexual terms, Frederic stops him immediately. When Rinaldi persists, Frederic responds:
Please shut up. If you want to be my friend, shut up.
Rinaldi understands Frederic's reaction:
All my life I encounter sacred subjects. But very few with you. I suppose you have them too.
This exchange with Rinaldi contrasting their feelings about sex shows the depth of Frederic's love for Catherine; she has become "sacred" to him. The contrast is further emphasized when Rinaldi explains that there are no "sacred subjects" for him; he feels no love or devotion as does Frederic.
In the final scene of the chapter, the contrast between Rinaldi and Frederic is further developed. Rinaldi makes an aggressive, unsuccessful effort to begin the old "priest baiting" with his companions. He fills his glass again and apologizes, saying he is "just a little crazy." He worries aloud about having contracted syphilis, but leaves to go into town where, no doubt, he will find another woman. Throughout this scene, Frederic plays the role of the peace maker, diffusing Rinaldi's aggression. He does not accompany Rinaldi into town; he remains behind to visit with the priest. As a foil, Rinaldi's life is one without purpose, emphasizing the changes that have occurred in Frederic's own life since falling in love with Catherine.