Hoi everyone _nightmare_wolf_  here! This is the info that you need to know!

Your name is Y/N and you have one sibling named frisk your parents were abusive towards you. But one day you didn't do as was told and they threatened to hurt frisk. That night you decided to run away from home with frisk and try to live on your own. For days you lived on the street until you were the right age to get a job. You worked until you had enough money to buy a house. As you and frisk got older you payed for a school so frisk could be social and have friends. One day frisk learned about the legend of Mount Ebott and the monsters. They wanted to set the monsters free so humans and monsters could live in peace. You-being the older sibling-of course said no to the idea and told them that it was way to dangerous. 

Frisk being frisk didn't listen and was way to determined to help the monsters. They snuck out while you were asleep and only left a note. You woke up the next morning and found a note on the table. It said

'Sis I decided To go to Mount Ebott and help the monsters. Don't try to come and look for me. Try and stay up there until I come back! I promise you that I will come back don't worry.

-frisk'

You cried for a couple of days before doing what frisk wished. So that puts you to where you are now.