Since you fell into the underground. Sans took you in..

The once friendly cheerful and lazy skelenton slowly became the more..well crazed..deppresed murderous skelenton..

Dust sans.

He kept remembering those dam resets and went on a frenzy..killing monsters..then the next day everything would reset like nothing happened. It was driving him nuts

He was on the couch zoned out..you walked to him and poked him. Causing him to flinch

His eye flashed that eerie red and blue colour. He looked at you with a forced smile..

"H-Heya kiddo.. didn't know you were up.."