The character Belle appears in the "Ghost of Christmas Past," and she is Scrooge's former fiancee. She breaks off their engagement because she realizes that Scrooge's obsession with money has made him a cold, heartless man. She wants to marry someone who is generous and kind, not someone who values wealth above all else.
It's important to remember that Scrooge never actually proposes to Belle in the story. He is already in love with money and has already chosen that path in life. She leaves him to find happiness with someone else.