Age: Late 20's or Early 30's ?
Grantaire was an Ugly drunk. He didn't believe in anything, and he was a party animal. He hung out with the ABC society. But said their dreams were blind and it couldn't happen. He didn't believe in the monarchy or the Republic. But in his heart he admired the leader, Enjolras. That was why he kept going to the Cafe Musain (where the ABC hung out and discussed matters).
He went to the barricade, and was told to go to sleep or leave. So he slept like a drunk. When silence broke out he awoke. That was when nearly everyone was dead. The national guard thought he was dead because he looked dead.
Before the National Guard shot Enjolras he said "I'm one of them too". He walked over to Enjolras and the yelled "Vive La Republique!". Then they were dead.
Sorry, that doesn't do Grantaire justice. But I'm getting eye strain, so this was very short.