The War of the Worlds
The Curate and the Artilleryman
The curate and the artilleryman are both useful (to us, to the narrator) because they provide different points of view on the Martian invasion. Because of that, we were tempted to call them each guides or mentors.
However, we think they might be better described as foils because, even though these two don't meet, their reactions and attitudes towards the Martian invasion are totally different. The curate falls back on God as an explanation and decides to give himself up for righteous punishment; the artilleryman falls back on the military as a form of survival and tries to keep himself safe above all.
What's really being contrasted here is not so much two people, but two different types of reactions to the invasion: the curate wants to submit to a higher power (God/the Martians), while the artilleryman wants to stay away from (or perhaps become) a higher power.
Mrs. Elphinstone and Miss Elphinstone
If we wanted to point to one pair of characters who could serve as the textbook example of a foil relationship, we'd point to Mrs. and Miss Elphinstone. If you have two characters, one of whom is brave and associated with dark colors, and one of whom is a nervous wreck and associated with light colors, you can be pretty sure that you have a foil relationship. (Check out "Characters: Mrs. Elphinstone and Miss Elphinstone" for more details.)