If Enjolras went to your high school, he'd be the really shouty kid who was constantly trying to get you to sign petitions and whom you secretly had a crush on, even though your friends all said he was "kind of … intense, right?"
In Les Misérables, he's the leader of the ABC Society, a radical political group that believes in democracy and the total overthrow of French royalty, no big deal. But he's not some miscreant from the streets who has nothing to lose by overthrowing the existing political order. In fact,
He was the only son of wealthy parents, a charming young man who was capable of being a terror. He was angelically good-looking, an untamed Antinous. (126.96.36.199)
Got that? Rich, charming, and good-looking—but he's still a true believer willing to sacrifice all of his advantages in life in order to do what he thinks is right. Like Marius, Enjolras is too young to have lived through the French Revolution. But he has read a lot about it and identifies with its cause so deeply that he feels like he was there: "He was familiar with every detail of that great event [The Revolution]; he had it in his blood as though he had been there" (188.8.131.52).
But perhaps the most important thing to know about Enjolras is that he always acts out of principle and never out of petty emotion. Even at the end of the book, he criticizes the French army for letting its soldiers die too easily, even though they are his enemies. "The idiots!" he says. "They're getting themselves killed and wasting our ammunition for no reason" (184.108.40.206). See? Enjolras has high standards even for his enemies. (This is why you could never actually date Enjolras—you'd constantly be letting him down. It would be exhausting.)
In the end, Enjolras gets gunned down for sticking to his beliefs and rebelling. But from what we can tell, he knew this was going to happen from the moment he picked up a gun. He also knows that the revolt won't be successful. For him, the symbolic act of defiance is more important than his life. After all, he thinks, what's the point of life if there's nothing you're willing to die for?