These two sets of foils are set up in part as impossible relationships. There's some sexual tension between both pairs, but in each case, it’s socially unacceptable and repressed. Both Clarissa and Septimus dwell extensively on their respective foils, though both of the relationships are in the past. The point is, these foils show the sides of our protagonists that they can't find it in themselves to express. Sally represents the wild side that Clarissa wishes she could embrace (running around naked, anyone?); Evans represents the heroism that Septimus entered the war to achieve.