Marius Pontmercy is one of the protagonists of Les Misérables. He's a bit of a dunderhead.
He's a classmate of Les Amis de l'ABC and occasionally assists them in their revolutionary schemes. He's from a rich family, but is poor himself and has very few friends. He's head-over-heels for Cosette. Upon her departure (due to Eponine), he finally makes up his mind about joining Les Amis, but only so he can commit suicide via French Army. Naturally, that doesn't happen.
He's the same in the musical, except he's with the ABC from the start.
He's probably the fandom's number-one lust object, which means he faces Mary Sues of all kinds, including those possessing Eponine or Cosette.