Once upon a time, a young child named Chara, carrying the Red Soul, climbed Mt. Ebott, a climb that legend said nobody ever returned from. She knew this perfectly well, and was glad of it. She needed to escape from the humanity she hated so fiercely, without care for how. She came to a massive pit, seemingly without a bottom, and as she attempted to circumvent it, she tripped and went tumbling down into the abyss...
When she woke she found a strange goat child had come to her aid and he told her of how his kind, the monsters, had been trapped underground long ago by humanity. He led her through the kingdom the monsters had created, back to his home, where she met his parents, King Asgore and Queen Toriel. The monsters were suspicious of Chara at first, given how her kind had imprisoned them, but over time they set aside their suspicion and she became like a second child to the King and Queen. However, she was truly more of a monster than any of them.
She poisoned herself and died, and Asriel, in his grief, absorbed her soul. She took over his mind and used the power of their combined souls to pass through the barrier that had sealed the monsters underground and planned to kill many humans so that the barrier could be broken completely and Chara could wage war on humanity. However, when Chara went to kill the first human she saw, Asriel fought back and prevented the murder. Although they were done no harm, the humans still attempted to kill the monster they saw and Asriel's body was driven back to the Kingdom, where he collapsed and died, both souls shattering.
Once upon a time, a young child named Frisk, carrying the Red Soul, climbed Mt. Ebott, a climb that legend said nobody ever returned from. She knew this perfectly well, and was glad of it. She never wanted to return to her life. She came to a massive pit, seemingly without a bottom, and as she attempted to circumvent it, she tripped and went tumbling down into the abyss...
When she woke, she found a living flower named Flowey had come to her aid. She was caught completely off guard when he tried to kill her and take her soul. Thankfully, a goat woman named Toriel saved Frisk from the creature, and told her of how her kind, the monsters, had been trapped underground long ago by humanity and King Asgore had declared war on the humans because of it, calling for any human who entered the kingdom to be executed and their soul taken.
Despite nearly every monster's attempts to kill her, Frisk never fought back. Instead she used a special ability she had discovered to persevere. She found her determination to succeed allowed her to come back from the dead, to reset the world at any time. Thanks to her determination she simply let the monsters' attack her and found herself making friends with many of them instead of fighting.
When she finally reached the barrier that needed a human soul and a monster soul to cross, she was forced to confront King Asgore. With all the friends she had made, however, Frisk was able to convince Asgore that the fighting was unnecessary. There were so many monsters gathered in one place, that W_H_E_N F_L_O_W_E_Y R_E_T_U_R_N_E_D A_N_D K_I_L_L_E_D E_V_E_R_Y L_A_S_T O_N_E O_F T_H_E_M H_E F_I_N_A_L_L_Y H_A_D T_H_E P_O_W_E_R T_O R_E_S_U_M_E H_I_S T_R_U_E F_O_R_M.
A_S_R_I_E_L.
Asriel, having achieved god-like abilities, tore Frisk apart, but once again, her determination allowed her to persist. She kept on dying and returning and dying and returning until she felt a presence within Asriel. Or rather, many presences. Frisk called out for her friends, she called out to Asgore, she called out to Toriel, and one by one their souls broke free of Asriel's grip. Then finally, Frisk called for Asriel. The one good soul left in this malevolent husk. As Asriel was freed, the husk fell apart, and from the explosion, the barrier to the outside world was broken. Frisk and the others left the underground forever. At least, until Frisk reset.
She was just curious. That's all it was. She told herself that after this was done she could just reset and put everything back to normal. She was just curious to see what would happen if she slaughtered every monster in her path. She became genocidal, killing everyone, all the friends she had made, even as they tried to help her. Knowing in the back of her mind that no matter what happened she could always reset. Except after she slaughtered Asgore and she slaughtered Flowey, she found she was no longer in control. A new being had taken over. She said that her name was Chara. Then she destroyed the universe.
For a long time, Frisk and Chara were the only presences left in existence, Frisk begging for Chara to restore the world and Chara not responding. Eventually, Chara realised she could earn something from this deal. If Frisk sold her soul to Chara, then Chara would be able to control Frisk at any time she liked. So she made the deal and Frisk accepted and the world was restored.
Frisk knew what Chara planned to do, and came up with a plan of her own. She went through the kingdom once against as a pacifist, laughing with her friends, having fun, trying to ignore the sins crawling on her back. She defeated Asriel, she saved him from himself, and then she stopped. She told everyone to go on through the barrier and she would follow them out. But she knew she couldn't leave. And she wasn't the only one. There were two people left in the underground, people with great evil just under the skin. So as a child named Asriel, carrying Flowey, journeyed back into the empty underground, a child named Frisk, carrying Chara went with him. After all, someone has to take care of these flowers.
