I think both these characters are kindred spirits who share their sufferings to together. Although Eva is privileged by being white, she suffers when her slaves suffer. Topsy understands that Miss Ophelia can hardly stand to touch her, let alone love her. As her reply, Eva touches Topsy and tells her she loves her. She asks Topsy to be good for her sake. "A ray of heavenly love" descends upon Topsy with Eva's touch and words of love. Topsy begins to weep and promises she will try to "be good." I think Eva has a celestial almost angelic role with Topsy.AS for a foil, Topsy's mischievousness and brutally honest understanding for the way things are acts as a foil to Eva's angelic like protestations for the slaves.