Ok ⚠️
This chapter contains lemons and smut throughout the story (not this chapter) so brace yourself

Sans POV
Just another day. Waking up to my brother Papyrus banging obnoxiously on my door, and of course not getting up until I force myself to since I have work at the lab. Of course, once I even slightly open the door Papyrus shoves a handful of bitter spaghetti down my throat. I head out to Frisk's to greet her like I do everyday, and find a golden locket in the shape of a heart on the way there. It was sitting in the snow. I picked it up and put it in my pocket. 'This looks oddly familiar...' I thought to myself. I kept walking to her house and knocked on her door once I arrived, and she greeted me with a sweet smile.
"Hey, Frisk?"
"Hello!" She greeted back.
"I made you a bagel."
"With-"
"Ketchup, ketchup." She interrupted and laughed.
"Thanks kiddo. Anyway, I found this golden locket sitting in the snow and it looked oddly familiar. Do ya... know anything about- Frisk? What's the matter? Does it belong to you or.." I said cautiously.
"Throw that out! Do not even think about that locket again!"
"W-What?"
"I-Just..! ERGH!!! Just trust me, ok?!!! That thing is dangerous! Did you open it?"
"No, what could be so dangerous about a gold locket? Christ, it's in the shape of a heart."
"...I can't tell you..."
"Oh uh... alright...? Well uh, I better get going. See ya."
She didn't respond.
-Time skip-
"Alph? I'm heading out early."
"O-oh, ok! Uhhhhh do you want any food before you leave???"
"Nah, but uh lemme ask you one thing- does this belong to you?" I held out the locket. Alphys paused for about 4 seconds.
"N-no, have a good evening, S-sans!"
"You too."
So, I walked outside and examined the locket more. Then, I made the stupidest decision of my life, but at the same time the best. I spinned it around in my fingers, and accidentally dropped it on the hard concrete, and it broke open.
"Oh... shit."
I picked it back up cautiously and saw a picture of what looked like Asgore as a child, and Frisk, but she looked slightly different. Instead of wearing her blue and magenta striped sweater, she was wearing a green and yellow striped shirt. Not to mention her eyes were open. But they were creepy.. they were red and glowing. The Asgore kid was wearing the same thing. I looked to my left and see a black soul. WHAT?! I could hear faint laughter from a girl coming from it. I cupped it in my hands and made sure it did not touch them and put the broken locket in my back pocket.
All the sudden a felt a tap on my shoulder. I turned around to see an incredibly attractive girl. It was the girl in the picture, but older. I'm the picture, she looked about 9, maybe 10. Now she looked 16 or 17. Her eyes, however, were black.
"I believe you have something that belongs to me."
I could feel my face heat up.
"U-uh.. oh you mean this?" I held out the locket.
"S-sorry, I dropped it and it uh... ehem... broke."
She looked disappointed and sad when I showed her the beat up locket. I felt guilt slither over my mind.
"...Somethings missing."
"Oh uh.. yeah there is a soul.." I showed her the soul that was now white and upside down.
"Huh-"
"You tried to steal him?!"
Him?
"Uh no, I just thought that it wasn't yours.."
The soul disintegrated right as I said that.
She broke down in tears on the ground.
"N-no!!! He can't... I... he..."
"Sh-shit... did I do that.. uh... I'm so sorry for your loss... I-"
"My name is Chara." She said tears still filling her eyes and streaming down her face.
"I need your help getting him back." She said.
"First, who's him?"
"Asriel. My adopted brother."
"The kid in the picture?"
"Yeah."
"And who is he?"
"Toriel and Asgore had a child before they discussed breaking the barrier. Their names combined are Asriel."
"Anything else?"
"Well, don't tell anyone I told you this.. but.. Asgore only needs 1 soul to break the barrier, but he needs 7 to break the barrier and become a god. Once he told Toriel, she divorced him for being so selfish and cruel."
"Th-that's terrible.. one last thing, why do you look so much like Frisk? If you know who she is.."
"Trust me, I know her well. I know everyone jut as well as Frisk does, Sans."
"You know my na-"
"Yes. I do. I lived inside of Frisk for a very long time. Controlling her, acting as her, being her, and eventually, this appearance was given to me. Before, I actually looked very different."
"I bet you looked gorgeous. Wait- I mean- not like your not pretty know or- I mean you didn't look pretty- wait no- ARGH. You bet what I'm saying, right?"
"Heh, yeah. Now follow me, let's to get my brother back."
She wiped her tears away, blinked and her eyes turned red. She was filled with
DETERMINATION.