(Note: I have assigned genders to both Frisk and Chara, identifying them both as female.)
Prologue: Someone Else
"Frisk, don't you have anything better to do?"
Frisk sighed. It was the phrase she dreaded, the phrase she had heard time and time again. It was the reason she kept reseting, the reason that she started over after everyone was finally free.
Perhaps Flowey had been right. She did want that "perfect ending". But it wasn't for herself. Frisk knew how to do it, and easily, too. She could live a perfect life with her new family on the surface in under three days' time.
All her needs were taken care of. She should have been happy, but she wasn't. Nothing could shake from her head the image of Asriel, weeping in remorse over what he had done to his family, what he had done to them all. Asriel, telling her to forget.
But she could not. She refused. She returned, again and again. Each time, she walked back to meet him. Each time, she hoped that this time, it might be different. That maybe, something she did differently might ripple, changing the timeline just enough to do it.
Just enough that he might finally accept her offer.
"Frisk, no! I don't deserve this! Look what I did to you! Look what I did to Mom and Dad! Just leave me be. All I can hope is that I can finally get some rest. At least I can be sure that my reset left with the other souls."
. . .
"...I know that, Frisk. But what about you? You have… friends now. You have… you have a new family. How can you give that up?"
. . .
"I don't know, Frisk, it's just not the same. You should know that. Besides, why should I matter so much to you, anyway? We've only known each other for maybe thirty minutes. Well, when I wasn't… you know..."
"More like thirty hours."
"What? That can't be right..."
"Asriel, I reset. I reset a lot. I went to the surface with the others, the first time. Actually, quite a few times. But every time I tried to settle down, I saw the same face. The face of someone I was told to forget."
"The one I couldn't save."
"Frisk, if I'm just the last piece of-"
"No, Asriel, you're not. I saw someone who had lost everything. Someone who did the right thing, and died for it. Then, when they turned around and fixed their mistakes, they sacrificed their own happiness for the sake of another.
"Asriel, this is a way for both of us to be happy. For everyone to be happy. You can have peace again, Asriel."
Frisk reached out with both of her hands, grasping his claw.
"Asriel, you can finally be free."