No, Éponine did not “save Marius”. Yes, she jumped in front of the bullet but he wouldn’t have been at the barricades in the first place if she hadn’t lead him there. Also, she was sure that he would die at the barricades. As she died, she told him that he was trapped, that he couldn’t leave and that he would die—that everyone was going to die and she was happy about it. She’s not a ‘martyr’, she’s not a 'hero’, she’s not a 'selfless person who died for love’. She died in some twisted plan to end up with the man she thought she loved in another universe. She didn’t save him, without her he still would have lived and he most likely wouldn’t have had quite as many emotional scars as he did having been at the barricades (though he might’ve been scarred still by the loss of les amis, Courfeyrac in particular)

She doesn’t need the whole 'martyr’ idea to still be an interesting character who still has some admirable qualities (I blame the musical, as much as I love it, they basically sucked away all of her mental illness which was quite obvious in the book and made her just a whole lot nicer and neater and prettier in my opinion). So no, she’s not a hero.

She’s still strong and a fighter, a survivor. She’s contemplated suicide, she’s thought about drowning herself, but she didn’t do it. She continued to live a depressing life, she continued to play watch dog for the police and be kept up all night because her hunger pains felt like knives inside of her. She was abused and belittled but she still had the strength and the hope to see that her world could become brighter. Her whole thing with Marius shows that, because she was in love with the idea that maybe he could whisk her away from the darkness and she could be happy and loved by someone and go to bed with a full stomach—sure, she didn’t go about this in a healthy way, but it shows that even with the shitty life she had, she didn’t give up until she convinced herself that she’d have everything she wanted in death. Even so, she survived years and years of abuse and malnourishment.

She’s still cunning and clever, even with no education at all, even when she could barely read. I’m not saying this was her shining moment as it certainly wasn’t, but she knew exactly how to lure Marius to the barricade, she knew that he would go if his friend wanted him there, she knew exactly how to lure him to what she was sure would be his death.

So, in conclusion, she wasn’t a hero or a martyr or a good person by most standards, but she’s not a useless character, she’s necessary to move the plot along, she’s got at least some redeeming qualities and most importantly there is so much more to her than her feelings towards Marius