Destitute street wench, desperate lover, professional piner-from-afar, lacker of several marbles, wannabe romantic heroine, unwilling criminal, unlikely savior, and all-around very strange young lady, Eponine Thénardier is one of the main characters of Les Misérables. According to Agent Maria Falcone's CAD, her species is "French." Make of that what you will.
Often depicted wearing a trademark big floppy hat, Eponine is a street girl who lives in the apartment next door to Marius with her family of criminals, whom she often serves as a lookout for. She's head over heels in love with Marius, but he's in love with Cosette, so she helps him out in any way she can in her own little stalkerish way. She creeps him out, but she's one of the closest things to a friend he has. After he decides to join the ABC to commit suicide by cop, she decides to do so as well, believing they will die romantically together. She can't go through with it, though.
She's the same in the musical, though we do get a bit more of her side of the story, which has inspired a lot of badfic.
She's beating off possession Sues with a stick. Alternately, she's a witch-with-a-b who does everything in her power to break Marius and Cosette up. The latter type often crops up to spite or parody the former, and really just serves to prove that Eponine is simply too complex a character for Suethors to grasp.