Papyrus is Sans's younger brother.
Papyrus appears as a much younger counterpart from his Undertale character. He doesn't wear his "battle body", but an orange sweater and darker orange sweatpants. However, he keeps his signature scarf. He also wears red shoes instead of boots.
He also has a huge crush on the protagonist and can be seen playing with his dolls of himself, Frisk, and Sans, who apparently was getting married to Frisk and, even after saying he wasn't invited to the wedding, said that Papyrus is his best man.
Sans: Sans is Papyrus's brother. They are very close, but Papyrus gets quite annoyed when Sans cracks jokes and puns. Despite being annoyed by him sometimes, he's always there when Papyrus is in need.
W. D. Gaster: Papyrus doesn't get to spend very much time with his father, since he is always busy working. Despite this, Papyrus still loves him.
Undyne: Undyne and Papyrus are best friends, often seen hanging out together.
Alphys: Even though Papyrus doesn't know Alphys that well, they are still friends. Alphys inspired Papyrus to start liking spaghetti.