A/N: this is both what I imagine Eponine thinking and what I would think if I was in this situation. Hope you enjoy!
Eponine had the letter. Marius had given her this letter. But the letter was not for her. It was a girl he fancied. Cosette. Eponine knew Cosette as more than a lover to Marius. The two had been like adopted siblings. As a child, she had hurt and abused Cosette. But now Cosette was doing the same to her. But was that fair? Eponine loved Marius since childhood. She had seen him around and fancied him. She and Marius got along amiably.
But she wondered. What if she never gave this letter to Cosette? The little brat had everything, why should she deserve yet another trinket. But no, she was being her mother. When Marius had first shown her kindness, she learned people were not inherently selfish and that the weak should not be taken advantage of. She learned this as she sunk into poverty. But how tempting, to perhaps lose this letter and have Marius love her instead.
She took the longest route giving ample time to think. Destroying the letter would not be so hard. There was a large river that the letter could easily be lost in the rushing water. There were gutters to drop it down. She could burn it or tear it up. She even considered just keeping the letter for herself.
The letter no doubt would contain confessions of love and passion for Cosette. She could easily pretend that Cosette's name was hers. She always longed to have someone love her and perhaps was so hungry for love that she would let herself pretend. She had pretended he was beside her. Eponine often imagined them living together and Marius wanting her affections. Eponine came from a rough family and a rough area. Pureness of heart was not a common phrase, and the streets were filled with the drunkards and thieves. She knew no one would blame her actions, she was after all a mere poverty-stricken girl.
But she knew she was better than that. Her mother often told her she was "Special" and "Compassionate" if only to taunt Cosette. She was cruel to Cosette as a child because most children mimic their parents. But deep down she had always felt bad for her.
Eponine took another lap around the town continuing to think. What would she gain? She knew Marius wouldn't notice she had again been dutiful in helping him. Cosette wouldn't be any wiser and Marius might die that night. She took the letter out of her coat and read the address. If Marius were to die, it wouldn't matter if she took the letter. But if he died she would hate herself, thinking it was his dying wish.
When he handed her the letter, it was a rude awakening to the reality of her life. She had known he fancied Cosette but like a child who still wants to believe in Santa, the fantasy could only go on so long. She then became angry, why was she wasting her time thinking about him or this insipid letter? She already had wasted the last few years caring for the boy and now a man who either didn't notice her affection or didn't care.
She drew near the home and stopped. There was a puddle, and Eponine saw her reflection. A girl who was dressed as a boy. She also saw a girl, who was fragile and tired. She had tried so hard to do right by her family and Marius. She was sad how her life had turned out.
Alas, she could not stand in the street forever and neared the home. She knew she could back out at any time and nothing would change. She was done with this battle. She became stoic, knowing that would be the nail in the coffin of Marius and her relationship. Her emotions were gone, she had no home and no hope. Her hope was always to be with Marius and live a quaint life and now it was to never be.
She was not evil and she knew this. She was not her mother or her sister. She would do the right thing for Marius, Cosette, and some higher power. She knew Marius may not see her devotion but her conscience could rest easy knowing she had not been cruel or mean. This was what finally made her decide to deliver the letter.
She knocked at the door. And a man stood there and asked her what she wanted.
"Monsieur, this letter is addressed to your daughter Cosette. It is from a boy serving the Rue Plumet," she said trying to keep from crying.
After she gave the letter and was assured the letter would be given to Cosette she ran away. She was now useless, Marius was out of the picture and she had no contingency plans. She finally decided to go back to the barricades. There was the slightest feeling this event would fail and she felt why not spend her last moments with Marius.