Belle appears during the sequence where The Ghost of Christmas Past is showing Scrooge his past. Here, we see that Belle was Scrooge's fiancée, but she eventually broke off their engagement due to Scrooge's growing obsession with money. Most adaptions depict the two having met at an office Christmas party when Scrooge was working for Fezziwig.
In the original story, after showing Scrooge the day he and Belle broke up, the Ghost also shows Scrooge that Belle eventually married and found happiness with another man; this scene is often skipped in the adaptations.
A Christmas Carol