Sans

Sans in Undertale (Or IS it..?) is a skeleton, known for how he messed up the universe "accidentally".

Appearance
Sans looks like his Undertale counterpart, except his hands are not connected to his body, and his body (except feet and head) are upside down, resulting in him having to move using his head.

Papyrus
Papyrus is Sans's brother. Unlike Sans, he is unaware that the "accident" messed him up too.

The Human
Sans is friends with the human, but can get suspicious at times.