Papyrus is a major character in Undertale. He and his brother Sans are both sentries living in Snowdin, a wintry settlement located near the Ruins of the monsters' original capital city. Papyrus frequently serves as comic relief due to his overzealous nature and general lack of intelligence.
After meeting Sans for the first time outside the Ruins' main entrance, Papyrus soon arrives on the scene to scold his idle brother for failing to "recalibrate" his anti-human puzzles in a timely fashion. He also reiterates his desire to capture a human, which Papyrus believes will allow him to secure a position of great prestige in Asgore's elite Royal Guard. The player character hides behind a lamp during the two brothers' conversation; once Papyrus leaves, Sans asks the player to humor his brother for awhile, as Papyrus has never actually seen a real human before.
Papyrus' traps and puzzles prove to be largely ineffectual as roadblocks to the player's progress. When the player character tries to cross a narrow rope bridge, Papyrus unveils his final "Gauntlet of Deadly Terror" consisting of swinging spike balls, spears, flames and an antique cannon. However, Papyrus decides not to actually activate this deathtrap, declaring that it would be "too easy to defeat the human with" before fleeing the scene. After traversing the length of Snowdin Town, Papyrus confronts the player character one last time on a foggy river bank and initiates a boss fight.
Before reaching Snowdin Town, Sans cautions the player about fighting his brother. Sans mentions Papyrus' "blue attacks," ostensibly referring to the previously-seen aqua-blue enemy projectiles that can only damage the player's SOUL while it is in motion. However, after several turns have passed during the boss battle, Papyrus alters the color of the player's SOUL to blue, introducing the concept of alternate "soul modes". While blue, the player's SOUL is affected by gravity, requiring players to press the Up directional key to jump over enemy projectiles. Players that manage to survive all of Papyrus' attacks will be given the option to Spare him and end the battle peacefully. If the player's HP is reduced to a single point during the battle, Papyrus declares the protagonist to be "too weak" and ends the battle prematurely; the player is then taken to a makeshift prison inside Papyrus' garage. Fortunately, the prison's poor design allows for an easy escape.
If the player has followed the game's "genocide" path up to this point, Papyrus will be reluctant to fight and instead offers to reform the player's homicidal tendencies. Although he will still initiate an encounter as usual, Papyrus immediately offers to "Spare" the player on their first turn; choosing "FIGHT" in this situation will perform an instant kill against Papyrus.