"The ruthless strategist of the royal knights. Not as evil as he seems on paper."
Papyrus is the ruler of Snowdin, and brother of Sans. He is second in command of the Royal Guard. He hates Sans sometimes and gets aggressive at times. In the pacifist route, Frisk might change everyone including him.
Papyrus shares many physical similarities to his Undertale counterpart. He is about three inches taller, at 6' 3", with good posture and bleached white bones. He has a scar running from his right eye, sharp teeth, and red eyes. Other than that, he looks like a relatively normal skeleton. His battle body consists of a faded red and black color scheme, with broad, spiked shoulder armor, red gloves, and black tight pants. His red scarf is the highlight of his arrmor, and almost acts as a cape when worn, as it drags behind him when he walks.
Papyrus is a cold ruthless Royal Guard commander and he cares little about the well being of other monsters, and will not hesitate to slaughter anyone just to set an example. He rarely shows any emotion, and lacks any remorse for his actions, no matter how gruesome, or merciless. Papyrus is perfectly fine with manipulating or torturing others to get what he wants. The only person he shows any sort of bond with is his brother Sans, and the head of the Royal Guard, Undyne. But Sans isn't safe from Papyrus' wrath, as their relationship revolves around their hate towards eachother. He verbally and physically abuses Sans. However he does love his brother but doesn't show this. He tries to toughen Sans up in the harsh world of Underfell. All this being said, Papyrus does not care about his merciless and will kill anyone who gets in his way. He is the very opposite of his Undertale counterpart.
Powers and Abilities
Papyrus is a lot more powerful than his Undertale counterpart, perhaps because he lacks the kindness his counterpart has. Like his Undertale counterpart, he can manipulate bones, which often glow a reddish color. Like Sans, he can also use telekinesis on a living soul. He is also shown to be able to manipulate gravity. He has complete control over his attacks, not unlike his Undertale counterpart.
Like in Undertale, Papyrus tries to capture Frisk but with death traps instead of puzzles, and without any help from Sans. Unlike in Undertale the traps are violent, and Papyrus attempts to lure Frisk into the death-traps, pulling them away at the last second.
As Frisk and Flowey attempt to escape Snowdin, Papyrus will block the way at the end side of the quiet town. Papyrus' fight is similar to the one in Undertale, only he can also use telekinesis like Sans. If Frisk spares him enough, he will knock them out and put them in his shed along with spaghetti, the key to get out, and his number so Frisk can call him as a "friend".
As it is stated above, Sans is the only other monster Papyrus seems to possess any form of bond with. Unfortunately, their relationship is largely founded on and primarily revolves around pain, bickering, and imtimidation, to the point where he makes Sans call him "Boss" as opposed to "bro". Despite all of this, at times it appears to be implied that he cares deeply for his brother, and is only so hard on him to protect them both.
Undyne is close to Papyrus, and one of his only friends. While their friendship is tense, like most other monsters of the Underground, they get along a lot better than other monsters do. He also shown to look up to Undyne as a role model.
(Note this is speculation as Underfell does not have a strict "canon" except their designs.)
True Pacifist timeline: Papyrus disliked Frisk and tried a lot of ways to kill and bring the human soul to Asgore, but failed, after Frisk stated that they believe that anybody can be a good person if they just try. Papyrus then helps Frisk by giving them spaghetti which heals 40 HP. Then Papyrus gives Frisk his phone number so that they can contact him at any time as friends.
Genocide Timeline: As Frisk ruthlessly murders every creature they see, they will run into Papyrus and Sans. Frisk will bypass every trap Papyrus adds to capture or kill Frisk with. Frisk will then come face-to-face with him and battle. Frisk and Papyrus will still have to fight like in the Pacifist timeline, but Papyrus will die in one hit. Papyrus believes he died in vain and says that they will still have to face Undyne, Asgore, and Mettaton before dying.
Papyrus doesn't like Flowey and Flowey doesn't like him.
Mettaton is one of Papyrus' few friends or maybe a little bit more than a friend.
ManiaKnight Underfell Game Differences
In the Underfell fan game, Papyrus owns a cat named Doomfanger, which Sans is forced to take care of. The relationship between him and Sans is milder, and his position in the Royal Guard is unmentioned as of version 1.0.2.
Primus Underfell Game Differences
Primus does not use any classic Underfell designs or canon, and is completely different to what is listed above.