Set as UT -> DR, same SOUL.
For long, the 2 species of this world, HUMANs and MONSTERs, have been seperated from each other, Humans in the much lighter Surface, & Monsters in the the contained environment, the Underground, beneath Mount Ebott. Ever since Humans won the war they started with Monsters, all of Monsterkind became trapped underground, living their lives and making the most of everything they had to replicate stars, sunshine and freedom... one day, it had to come. All they needed was their king Asgore to collect 7 human SOULs to break the barrier...
...Six humans' lives had been taken already, and the last one they needed would be another who fell into the Underground... Frisk. A human who traveled the entire Underground, from the Ruins to Snowdin to Waterfall to Hotland to the CORE to New HOME, where King Asgore dreaded their arrival... but, during their entire adventure, while mostly unexpressive... they were kind & compassionate to all, including the former Queen & their temporary Ruins caretaker Toriel, to the humorous Skele-bros in Snowdin, even to those who stood in their way & tried to kill them, like the loyal & passionate Undyne, all their acts going against what a certain sadistic flower told them at the start of their journey, "It's KILL or be KILLED.
...but after all that, having learned the true side of everybody, all of Monsterkind stood with them at their last encounter with that same flower... and got them to reveal their true self: the King's late son, Asriel. Even with the godly powers that came with the absorption of the 7 human SOULs and every Monster in the Underground, the human Frisk would reach out to them and SAVE everyone. And now, as one last action with everyone's SOULs to repay for everything they've done... Asriel broke the barrier. Monsterkind... was free, and all of them thanked the kindness the human Frisk showed. With all of them ready to live in peace with Humans once more, Frisk themselves was ready to go home...
While Frisk themselves was kind & understanding, despite their age, the actions they made seemed to have come from another source... the SOUL inside them. Nearly every choice made felt as if a voice in their head told them what to do... and Frisk was obedient to them, expected for their age. They seemed kind after all. Except now, Frisk was free. Frisk was now... Frisk. Everything they did... was Frisk.
...despite everything, it's still you.
...now what? Well, Humans & Monsters are living in peace again. All those Monsters you had come to knew are now living their own lives, like those in the nearby Homrtown. Of course, you can still contact everyone you met before, Toriel gave you your first cellphone after all! It's just you haven't in a while, ever since you told Toriel you wanted to go home instead of live with her on the Surface. Now, you're living your own life too.
Waking up, eating what your human parents make you, going to school, and choosing what you want to do. just like any other human. It's been a couple years since all that happened, but you're still young, curious, and hopeful! So, as you were walking home from school today, you wondered what would happen today!-
SWISH!
All of a sudden, you felt the collar of your shirt grabbed by someone, who pulled you into a nearby alleyway. This human... was a teen, a bit older than you, and, frankly, a bit similar in appearance to you, even a (differently-colored) striped shirt.
Kris: You... I knew you couldn't be far. Listen here, kid...
They put you down. Their tone sounded dead, but slightly desperate.
Kris: ...I need your help. I know you dealt with something similar... how the hell do you deal with this?
They pointed at where their heart would be.
Kris: ...This... this parasite... did it bother you? How... clenching their shirt, almost as if they were about to rip out their heart How the hell do you get RID of it?!... realizing ...Sorry, I'm... going too fast. I haven't even said my name yet...