Frisk.
Despite how hard he tried, Frisk was never able to find happiness, he always retained a face devoid of emotion, negative or positive. Reset after reset he had become just as nihilistic as Sans was before going to the surface, and the only thing he was able to feel anymore was frustration and apathy. Frisk had not showed any joy since the first few pacifist runs, after all why bother if everything was just going to reset anyway? The worst part of the whole ordeal was that the few people who retained their memories after each reset believed it was Frisk who reset the timelines after each "happy ending". He would find fulfillment with a new family and friends he never had before falling into Mount Ebott, he would build new memories like his times with his mother Toriel, Alphys and Undyne's wedding, Sans' jokes and his first date since he left the underground. Then, one day without warning he would go back to the beginning, and Sans would blame him for it each time it happened. The first time it happened Frisk was playing in the flowers of Mount Ebott with Toriel, then within the blink of an eye he was lying at the bottom of the same pit he had first fallen into when he came to the underground. The second time happened after he saved Asriel, he was on the surface for almost a year, then while hugging his brother who's family he was adopted into he found himself holding nothing, back in the same dark pit, alone. Time after time Frisk tried everything he could to get his happy ending, achieving that one thing became his soul purpose in life. Frisk even brought back Chara in hopes that it would fix his problem. In the end though, Chara himself was not responsible for the resets, and was as much of a puppet as Frisk was, another toy to be played with by someone else.
Frisk sat down on a rock overlooking the city at the base of Mount Ebott. The sun was setting, covering the sky in a golden hue while a warm wind blew through the mountains, causing the golden flowers decorating it to dance with each gust. All these beautiful sights were lost on Frisk though, he simply stopped caring at this point and pondered, his face unemotional as always but his heart filled with annoyance and frustration. He had relived this same moment many times though each time it was a little different. Usually, it was Sans who visited him here, striking up a conversation and trying to talk Frisk out of resetting, even though he had no control over it. Other times it was Toriel or Papyrus who had visited him, wondering why he looked so glum, but Frisk would always lie to them, saying he was simply admiring the sights. One time it was Flowey, asking him if he had anything better to do, apparently Frisk did not. This time however it was Chara, his own twin brother who he had helped revive.
"Greetings Frisk." Chara said in his usual emaculate tone as he approached his brother and took a seat next to him on the grass. Frisk didn't bother turning to face him, he kept his same position as if he were a statue and never spoke a single word. Chara's life since he became human again had improved greatly, Toriel and Asgore forgave him despite everything he had done and he even came to live with them and Frisk. However, Chara's actions when he was "Chara the Demon" have left him with a sea of regrets once he was finally able to love again, it was only through his best friend and adoptive brother Asriel that he was able to pull himself together and move forward. Chara sat quietly next to Frisk for several minutes, looking out at the city below before finally speaking.
"Beautiful, yes? All of this was built in only a few years." Chara said as slowly the lights in the city of True Home turned on, Frisk didn't speak, he only turned to him briefly and then huffing in annoyance.
"Did you come here to guilt trip me about the resets?" Frisk said, Chara simply sighed.
"No. I know you have no control over them, it's the only explanation as to why they keep happening." Chara said sadly.
"So you know I can't stop them?" Frisk asked, turning briefly to Chara. Chara laughed sadly before sighing.
"Yes. But that's not why I'm here." Chara said as he began to play around with his locket.
"I wouldn't be here if it weren't for you, you saved me even if I didn't deserve it." Chara said with a smile.
"Funny, isn't it? When I was human before I took it for granted, I hated humans and I hated being a human. But then when I became a demon I completely lost sight of who I really was, I did... terrible things to you all. I guess the true problem with me all along is that I wasn't human enough." Chara said as a few tears fell from his crimson eyes as he opened his locket, looking at the picture inside.
"Toriel, Asgore, Asriel and especially Sans should have destroyed me, or at least drove me away for everything I did, but they... forgave me. They took me in despite my terrible decisions and helped me try to become a better person. I just-" Chara spoke but was soon interrupted by Frisk.
"Shut up!" Frisk said, he was angry and crying, his hand gripping his forehead and squeezing hard. "Just... shut up!"
Chara fell silent for a moment, looking down at the ground.
"I wanted to say thank you." Chara said sadly, as Frisk's hands shook as he cried in frustration and grief.
"Thank you for what!?" Frisk asked angrily, his voice shaking. "For saving you? For saving Asriel? For freeing the monsters from the surface!? What difference does it make when everything's going to reset!?"
Frisk stood up from the rock and started to storm off, but Chara followed him closely.
"It matters!" Chara shouted back at Frisk.
"No matter how often time resets I can't fully cleanse myself of the terrible things I did, but the same applies to you, just because time resets doesn't truly undo the fact that you helped everyone, that you brought them happiness even if it was only for a little while!" Chara angrily yelled back at Frisk, who stopped in his tracks and leaned against a lone tree and started to cry.
"Chara, I just want this to end already..." Frisk cried, Chara had hardly seen Frisk showing so much emotion.
"The resets, crawling through the underground, having to save or destroy everyone all over again, I just want it to stop..." Frisk said as he slid down the tree, his face buried in his hands.
"It's been nearly 5 years now, this is the furthest a timeline has ever gone, but I know that one day it's going to reset again, but I'll never know when... I can't handle this anymore, I just can't!" Frisk said as he began to sob, soon finding his brother next to him, rubbing his back.
"I just want to go home, to keep my friends, to live with Toriel and Asriel and... you as well." Frisk sobbed.
"It's okay brother, maybe there won't be another reset." Chara said as he hugged Frisk. "Maybe whoever was resetting finally got tired of the game."
Chara comforted Frisk for a while, it was a strange experience for both of them. Frisk was usually the one to comfort others, he had comforted Asriel and Chara when they "returned" and he helped his other friends during rough times as well, but now it was he who needed it. Chara on the other hand never cared about anyone other than Asriel before now, but he had felt truly sorry for Frisk and what he was going through, and knew that the secret he was keeping from Sans and Asriel was too much for him to tell. To destroy their last ounce of hope by telling them that someone he doesn't know, that he can't even see or stop is responsible for resetting over and over. Suddenly, he heard Asriel's voice from the other side of the hill they were on.
"Frisk! Chara!" The monster goat boy called out as he raced over to them, by now Frisk had dried his tears and was in a better mood.
"Whatever happens Frisk, we'll try and find a way to stop the resets forever." Chara said as he patted him on the shoulder, Frisk smiled for once in a long time.
"Golly Frisk, you feeling okay? Your eyes look kind of red." Asriel asked, Frisk shook his head and rubbed his eyes.
"I'm good bro, just allergies." Frisk said happily.
"Alright, let's go play before it gets dark, mom will be making supper soon too!" Asriel said as he ran ahead of them. "Race you guys!"
"Hey, wait up Azz!" Frisk said as he ran after him, Chara following shortly afterward.
"You're not fast enough to beat me!" Chara said as he ran ahead of Frisk.
"Oh yeah! We'll see about that!" Frisk said as he ran forward before his foot caught a branch and he tripped, landing face first on the ground.
"Ouch! I'm okay guys, I just..." Frisk spoke, but when he sat back up he was somewhere dark, with only a single ray of light shining down on him from above. Chara and Asriel were nowhere to be seen, he was underground once again.