In the beginning, Marius is a young, lovesick revolutionary. He’s little more than a child. At some point, he grows up. His growth begins with the death of Eponine. (Somehow it’s loss that grows us the most). His growth continues when he awakens after his rescue from the barricades and discovers that his other friends are all dead. He struggles with survivor’s guilt as he recovers from wounds he received at the final battle. Cosette and Valjean are there to help him. But it’s only when Marius is close to recovery that Valjean tells him the great secret he’s kept from Cosette.
This is the point where Marius from the play and Marius from the book take turns down different paths. I think the writers of the musical knew we’d be furious with Marius if he took after the book, so they created a character we would respect. In the book, Valjean tells Marius who he is. Marius is newly married to Cosette and Valjean feels it’s proper for her new protector to know what sort of a person her father is. He asks Marius to decide what sort of part Valjean should play in Cosette’s life. Marius has a very Javert-like reaction. (In Marius’ defense, he doesn’t know that Valjean was the person who rescued him from the barricades).
Marius agrees to Valjean’s proposal to stay out of Cosette’s life. Knowing that Valjean is a convict causes Marius to bury the money that Valjean gives them as a wedding gift, refusing to use it because he thinks it’s been stolen. Valjean, without Cosette in his life, loses all will to live and slowly fades away.
In the play, Valjean fears Javert is still coming for him. He wants to pass Cosette off to Marius as quickly as possible and disappear so that his disgrace will not affect Cosette. Marius protests, but he understands. It’s the protesting that sets him apart from the Marius of the book. It’s Valjean’s love of Cosette that causes him to separate himself from her. Because Cosette is finally safe, Valjean allows himself to rest at the end of his life.
But we aren’t at the end quite yet.