Papyrus, from the Undertale series, is a spaghetti loving, self-absorbed skeleton and is the brother of Sans.
Note: Due to undertale's branching paths, it has been divided into 3 parts for the sake of it not being a huge block of text.
Sometime before the events of Undertale, Papyrus and his brother showed up one day in Snowdin and asserted themselves. Sometime after that, Papyrus became friends with Undyne and began training with her.
Papyrus first appears in Snowdin Forest, where he arrives to check up on his brother. After learning that Sans had not made any progress over eight solid days, Papyrus, ignorant of the human hiding nearby, berates Sans for not recalibrating his puzzles. He then delivers a long tirade to Sans that he will capture a human and become popular. Further in the forest, Papyrus meets the protagonist for the first time, though he initially confuses them for a rock.
Papyrus attempts to antagonize the protagonist with puzzles and japes, but he eventually warms up to them. He almost refuses to fight the protagonist out of pity. Though Papyrus initiates an encounter, he never kills the protagonist and instead places them in his shed if their HP reduces to one. The protagonist can easily escape his shed.
Ultimately deciding that the protagonist is friendly, Papyrus lets them continue into Waterfall, advises them on how to exit the Underground, and offers either a hangout or a date back at his house.
Returning to Papyrus's home after Papyrus's fight initiates the hangout or date. During the date, the player can press C to open the Dating/Friendship HUD, which consists of a clock, a radar that tracks the Annoying Dog, a level of crime graph, a bar with "REEL IT IN!" above it, a population chart, and an egg. The date shown on the clock matches the day on the player's computer, and the graphic next to it changes accordingly based on the time of day the player's computer provides. If the player opens the "dating HUD" before Papyrus reads from the dating book to open the Dating HUD, he realizes it has already been done. However, if they do not open the Dating HUD after a certain period of time, Papyrus moves on and the "dating HUD" is not present for the date.
Papyrus dresses in his "secret style" and hides a special gift under his hat for the protagonist. Ultimately, Papyrus decides that the protagonist cares too much about him for their own good, and sends them back on their way with his phone number, which the protagonist can call at any time to hear his opinions about the room that they are in.
When the protagonist makes their way into Waterfall, Papyrus attempts to sway Undyne from killing them; Undyne refuses to yield, however, and Papyrus finds himself obligated to help her. He calls the protagonist and asks them to confirm what they are currently wearing. Papyrus is loyal to both Undyne and the protagonist by employing a ruse that backfires regardless of what the protagonist does.
If befriended, Papyrus calls the protagonist while Undyne is chasing them and say that they should all hang out. Backtracking to Undyne's house during the chase leads to the protagonist discovering Papyrus already waiting outside her house. After the protagonist chooses whether or not to give Undyne water in Hotland, they can go to Undyne's house, and Papyrus offers dialogue based on the protagonist's actions after his call.
He flees from the hangout early on, after using reverse psychology to get Undyne to befriend the protagonist.
Befriending Papyrus is necessary to finish a True Pacifist Route. Flowey guides him to call all the protagonist's friends to New Home. Before Asriel's boss battle, Papyrus rushes to intervene with the battle between Asgore and the protagonist. He then meets Toriel for the first time and becomes annoyed once she tells him a pun.
Flowey then restrains Papyrus and the rest of the monsters present with vines. Papyrus defends the protagonist from Flowey's bullets and states his faith in the protagonist.
Papyrus discovers that there is no need for the Royal Guard anymore and proclaims that this is "THE WORST POSSIBLE ENDING." He also comments that Toriel is a "HUGE DORK."
In the credits, it seems many of Papyrus's dreams have come true: he drives a red car along a highway, something he has always wanted to do. Later on, Asgore is shown trimming a hedge in the shape of Papyrus's face, something that Papyrus wished would happen once he became a famous royal guardsman.
In a Genocide Route, Papyrus pulls much of the same antics that he does in a Neutral/Pacifist route, except often cites the protagonist's unwillingness to complete his puzzles. He remains oblivious to the protagonist's motives until their battle, where he instantly spares them, wishing to guide the protagonist on a better path and be friends. The protagonist can then spare or kill Papyrus.
If the protagonist kills him, he states he still believes in the protagonist and thinks that they can become a better person. If they decide to spare him, Papyrus proclaims that they "DIDN'T DO A VIOLENCE," and says that he was secretly terrified of the protagonist. From that point on, the protagonist may choose to "hang out" with Papyrus, having aborted the Genocide Route by sparing him.
As Mario is walking, Papyrus walks in, using a fake dance that is sexy enough to paralyze a human, but is just a joke by Sans. After learning that the dance was fake, he asks a question as a puzzle: 1+1. Mario can't figure it out and is captured due to his stupidity. After Papyrus learns that Mario loves spaghetti-lover, he tries to save him from the po-po.
Papyrus was seen among the T-Pose Zombies in Waluigi's army.
Papyrus appeared in a flashback.