Who is Naruse Mitsuko from Deep River and what is their importance?

When Mitsuko is a college student, her friends call her Moīra, after a character in a novel who seduces a young man who is too good. Mitsuko meets an awkward young male student named Ōtsu, who is a Christian and who goes to a chapel to pray every day after school. Mitsuko decides that she will pit herself against God and seduce Ōtsu away from the Church. Mitsuko gets Ōtsu to give up praying at the chapel to be with her, and then she dumps him.