It's been around half a year since your friend, Frisk, as you recall, managed to do something that broke the barrier and let the monsters live with humans. You weren't around in your hometown and neither Mt. Ebott to witness it, but you didn't care really. What was important, is that you returned from the trip that lasted what seemed a lifetime.
So, you wanted to see what really happened. Was that a good idea? I'd say, debatable..
Boy, something told me I should've written this earlier!
So, uhm, an short brief.
YOU here are a MALE, 16 years old.
CHARA and FRISK here are female.
CHARA is 16 years old, too.
FRISK is just a year younger than you.
I am sorry to disappoint any readers that see the story the other way, but this is how I see it.
Undertale belongs to Toby Fox
And you uh.. Belong to you.
Y/N was the eldest of the royal family. Her life was great. She was turning 16, the age where she'll be able to help her parents with royal duties, and get more privileges, when her younger sister, Frisk, goes missing. Y/N's life falls apart, while her mother heartbroken, and her father started to ignore her. Not having a great life, she decides to run away, learning about the hard life of being a citizen. Having to live on the complete opposite side with poverty, she learns she has to hide, and everyone isn't what they seem. Not even her.
This is my first story, so please don't judge I know it's crap so... It's an Undertale Sans x Reader P.S. I don't own Undertale. Undertale is owned by Toby Fox