We'll just say it: the romantic hero is always the least interesting character in a story. Give us Enjolras or Thénardier over lovesick Marius any time. Instead of being passionately fired up for revolution or being on the make to survive, Marius comes into this book for one main reason: to fall in love with Cosette.
But before he meets Cosette, the novel needs to provide him with a backstory, and what we learn is that Marius grew up never knowing his father because his rich grandfather despised the man and threatened to cut Marius out of the family inheritance if the dad didn't disappear. Over time, Marius comes to learn that his father was a war hero who always loved him and performed all kinds of heroic feats while fighting for the French army under Napoleon.
Okay, so it does start to get a little more interesting here. As Marius finds out more about this stuff, he feels that "The rehabilitation of his father led in the natural course of events to the rehabilitation of Napoleon; but this, it must be said, did not come easily to him" (220.127.116.11). In other words, Marius' increasing interest in his father makes him increasingly interested in everything his father fought for: namely, the spread of democracy and revolution.
Even after Marius gets himself kicked onto the street for defying his grandfather, he compares himself to his dad and thinks, "What were his own hardships and misfortunes, compared with the colonel's heroic life?" (18.104.22.168). Now, Marius uses his father's memory as a sort of North Star to guide him through life.
Marius' biggest concern before meeting Cosette is the memory of his father and his conflict with his grandfather. Sounds snooze-worthy? It gets even more tedious when he meets Cosette's in a public garden and falls heads over heels in love with her: "What message was to be read in her eyes? Marius could not have said. Nothing and yet everything. A spark had passed between them" (22.214.171.124). (We wish that spark would hit us, since we can't summon up much interest in this pair.)
And then, just to make things really eye-roll-worthy, Marius falls into cliché despair when Valjean pulls the disappearing act and Cosette seems lost forever: "He had ceased to be the hot-headed dreamer of dreams, the bold challenger of fate, the youthful builder of futures, his mind teeming with castles in the air. He was like a stray dog, plunged in black despair" (126.96.36.199). In other words, Marius has so little character that, without Cosette, he becomes a black hole of boring.
Luckily for him, if not for us, Marius and Cosette eventually find their way to one another and get married. And as far as we know, they live happily ever after.