Who is The Patient's fiance from The Screwtape Letters and what is their importance?

This girl is particular offensive to Screwtape. Screwtape positively hates her. While Screwtape's attitude toward the male "patient" is the rather detached attitude of someone studying a specimen, Screwtape's passions are truly provoked by his fiance. From this, we can understand she is an extraordinarily good person. Indeed, she is full of virtues.