Disclaimer: Definitely mine. That was a lie.
Grantaire loves all his friends, the little shits, but he has a special place in his heart for Eponine. She understands him on a level that not even Enjolras, the man to whom he bares his soul through drunk-texts, or Jehan, the poet who sees things in people they themselves don't see, do. She has the kindest heart, the saddest voice, and the kind of inner beauty that only comes from extreme hardships and manifests itself in one's eyes and smile.
She's kept him from throwing himself off of a building more than once, and when he stumbles into their apartment, rejected by Enjolras and drunk beyond belief, she merely kisses him, helps him into her bed, holds him as he cries, and wakes him up the next morning with coffee and bacon.
She's the only one of the Independents who can match him at a drinking contest, those she's never defeated him yet. She drinks with him, she cries with him, and she is always there for him.
Marius claims that "'Ponine" came from him, but everyone really knows that it's the affectionate nickname Grantaire gave her. Marius thinks that the reason 'Ponine is there is for him, and maybe it started off that way, but now everyone knows that Eponine is there for Grantaire more than anyone.