Papyrus

Papyus in Undertale (Or IS it..?) is Papyrus, glitched by Sans's "accident."

Appearance
Papyrus is most likely the most glitched, having his left eye and right foot upside down, a 3rd foot coming out of his right arm, and a 3rd arm coming out of the side of this head.

Sans
Sans is Papyrus's brother. Unlike Papyrus, Sans is aware of the accident and is trying to fix everything.

The Human
Papyrus is friends with the human, despite trying to capture them. He doesn't remember the human's name due to a loss of memory during the accident.

Undyne
Papyrus used to know Undyne, but like with the human, forgot her name during the accident.

Trivia

 * The reason why Papyrus cannot remember the names of Undyne and the human, and is also unaware that he's affected by Sans's accident is because his brain suffered a great loss of memory when he was glitched. He still remembers Sans and how to cook spaghetti, however.