Who is Belle, and why was she important to Scrooge?

Belle was the love of Scrooge's life. They were engaged, but Belle broke off their engagement. She told Scrooge that she couldn't marry him because she'd been replaced by something he loved more.... gold.

“It matters little,” she said, softly. “To you, very little. Another idol has displaced me; and if it can cheer and comfort you in time to come, as I would have tried to do, I have no just cause to grieve.”

“What Idol has displaced you?” he rejoined.

“A golden one.”