Disclaimer: Not mine.
The first time Montparnasse's mother found Eponine asleep in his bed, she gave them a talk about safe sex. After the next few times it happened, she gave her a key. Eponine had always been the only girl who had never looked twice at him, and maybe that's why he was so damn obsessed with her. He denied it to all his friends, he did, but he knew that he would do anything for her, that dirty, impossible spitfire. He had taught her to smoke her first cigarette, to drink her first beer, and he'd taken her virginity one night in the back of her own Chevy Impala.
For the moment, she was traipsing after Pontmercy and acting all high-falutin' with the pretentious douchebag Independents, but it didn't bother him too much. The bond that they had transcended infatuation. This had happened before, where she would get caught up with another man or woman, and she always came back to him in the end.
In the meantime, he always had Jehan.