After you got adopted when you were little because your family died, you lived somewhere close to monster. Of course, you made friends with a skeleton. He hung out with you more than his friend it seemed.
Also, Before Sans, You were friends with a human named Chara, There were your friends since you were little. But people say that 'he' (Yes, Chara is a he here) Jump down Mt. Ebott. He was your only friend, couple Mounts later. Frisk, Your other friend, also jumped Mt. Ebott They must've wanted to save Chara. Frisk was probably two late.
Then, She brought all the monsters up two the surface, no sign of Chara. Chara probably died, you lost hope he was alive, then a skeleton herd you crying, you were at a bridge, you wanted to die, then when you fell, the skeleton saved you, and you two become good friends! You and that skeleton started meeting up there two talk. You always moved places cause your parents kept finding new jobs, you were afraid if you had to move away again.
You had to move again, your parents had a new job somewhere. You didn't wanted to leave Sans and Frisk, they were so nice to you. Except for the bullies who bullied you a lot.
【Years later】
It seems you had to move back there, only cause your parents found a really good job over there, plus, they really like the house, so they decided to move back there. It wasn't for long you found your skelly friend.