The answer is the sentence "Gerasim alone did not lie; everything showed that he alone understood the facts of the case and did not consider it necessary to disguise them, but simply felt sorry for his emaciated and enfeebled master. " A character foil is defined as a character who contrasts with another character (usually the protagonist) in order to highlight particular qualities of the othercharacter.
Gerasim is Goodness personified. In the desert of falseness and fakery that is Ivan Ilych's world, Gerasim is the lone oasis of genuine honesty and kindness. Besides young Vasya, he's the one character who is obviously not false.