Major Sergius Saranoff and Major Petkoff serve two major roles in Arms and the Man by George Bernard Shaw. They serve first as foils to Captain Bluntschli and second as obstacles to the romance between Captain Bluntschli and Raina.
The central plot of the play is a fairly simple romance. Raina and Captain Bluntschli meet and fall in love after Captain Bluntschli scales a drainpipe onto Raina's balcony while trying to escape the Bulgarian soldiers searching for Serbian stragglers. The first obstacle to their relationship is that Raina is engaged to marry Sergius. Secondly, Major Petkoff acts as the stern parent who might prevent his daughter from marrying what appears to be a poor mercenary, although the "chocolate cream soldier" eventually turns out to be a wealthy hotel owner, making all objections vanish.
Both the Bulgarians also represent an aristocratic and heroic ideal, grounded in a romantic belief system and provincial cultural milieu and thus act as foils to the pragmatic, cosmopolitan bourgeois mercenary.