Belle is Scrooge’s former fiancée. She was visited by Scrooge with the Ghost of Christmas past. When Scrooge sees Belle, he is reminded of his greed. Because he loved money more than love, he lost Belle and therefore he lost the only happiness he had in his life.
In the section of A Christmas Carol with the ghost of Christmas past, we learn that Belle was once Scrooge's girlfriend. She rejected him because he changed too much. She tells him:
"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.”
In other words, the love of gold or money has replaced Scrooge's love for her and so she breaks off their relationship. Scrooge later sees a vision of Belle who has married someone else and is surrounded by loving, happy children.
I would like to add that at the time that Scroog was engaged to Belle, he did not so much love money, but was afraid of poverty.
Scrooge sais to Belle "there is nothing on which it is so hard as poverty; and there is nothing it professes to condemn with such severity as the persuit of wealth!"
Belle was Scrooge's fionce. Scrooge said that she was the only person he ever loved.
Some of the new movie versions of A Christmas Carol make Belle Scrooge's sister.