Relationship with Victoria
We first see Lord M in Doll 123. He offers his services as an advisor to Victoria, and at first she refuses. Victoria and Lord M's relationship becomes strained after Victoria realizes he is the only man she would ever want to marry. Lord M and Victoria eventually part ways after she marries Prince Albert.
At one point we learn that Lord Melbourne was once married and had a son. After his son tragically died, his wife left him and he never saw her again.