No no no but what if Grantaire has an older brother that he doesn’t tell anyone about because he’s always felt so damn inferior to him. Like, his brother is movie star hot, always did well in school, captained sports teams, all the girls loved him; hell, all the boys did too. And then there’s Grantaire with his mismatched face and his cynicism, alcoholism, poor life decisions and unstable career choice. He’s always felt like a failure in comparison.
But then he waltzes into the Musain unannounced one day and tacklehugs Grantaire and they all want to know “WOAH, ‘Taire, who’s the supermodel?” and he has to introduce them. And he loves his brother, he really does, but he just knows his friends are going to love his brother more than they love him - I mean, look at him - and he doesn’t want to lose them to him but at the same time he thinks they deserve better, so he withdraws into himself while everyone fawns over him.
Well, not everyone. Because Enjolras doesn’t get it. Sure, the guy’s alright, he’s funny, holds a good conversation, but he isn’t Grantaire. And he doesn’t understand why everyone’s basking in this guy when Grantaire is right there; radiant, witty, incredibly intelligent and cunning and strong and passionate Grantaire - and he’s always been there; why does nobody acknowledge that? So Enjolras, instead of being overwhelmed by this sudden wave of emotion that he doesn’t really know how to deal with, goes to sit with Grantaire, who doesn’t hide his surprise well.
"Not striking up a golden boy alliance, then?" and he only sounds a little bitter. "You two would be unstoppable together." And Enjolras frowns, and he doesn’t even have to think before he says "Why should I? I’ve already got you; I’d rather have you." And maybe that’s all Grantaire needs to hear, because he gives Enjolras the first real smile since his brother appeared through the door and starts joining in with the revelry around them instead of moping.
[And maybe later on, after Combeferre has tugged the brother aside for a quick word, he comes over to speak to Enjolras and gives him a spiel along the lines of “I’m not here for long and I think it’s going to take you a while to realise just what’s going on, so I won’t be there when you get your act together. Because of this, I’m gonna jump the gun and warn you now: you hurt him, you break his heart, and you’ll wish you were never born.” And Enjolras nods, but he disagrees in part; he has already realised, and he would never break Grantaire’s heart - because doing so would break his own.]