Papyrus

Papyrus (known as COOLSKELETON95 on UnderNet) is the younger one of the two skeleton brothers, the other being Sans, and a major character in the story. Befriending him is necessary to finish the True Pacifist Route. He wants to catch a human, so that he can become a member of the Royal Guard.

Appearance
Papyrus is a tall, perhaps anatomically inaccurate skeleton with a skull shaped in a more straight and geometrical structure (in contrast to his brother's more rounded skull). His mouth is in a smile shape most of the time and his eyes are pupil-less, resembling vertical slits with his expressions being formed mostly by his skeletal pseudo-brows.

He wears a custom-made costume known as his "battle body", which is made up of a white torso piece with various golden trims (and embroidered with a symbol that very heavily resembles that of the Starman from the Mother series), a rough red scarf or cloth around his neck, red gloves (trimmed with gold), blue briefs with a golden belt, and long red boots.

Personality
Papyrus is a loud and flamboyant skeleton who presents a confident, charismatic image of himself. He tends to work hard, and despite his brash personality, he is a very kind skeleton at heart. He is extremely optimistic and innocent, as shown on a Genocide Run when he continues to believe the protagonist is good at heart, even as the protagonist attacks and ultimately kills him.

Papyrus is occasionally rather oblivious and nonobservant, such as when he tries to rub different kinds of creams and "sauces" behind his ears, only to realize that he doesn't have any ears. He can be witty at times, especially when he used reverse psychology to trick Undyne into making friends with the protagonist.

For all of his skeleton faults, Papyrus seems to match this with how overwhelmingly determined he is, a feature that made Undyne take notice of him when he waited and begged her from midnight to morning for her to accept him into "royal guard training".

He really likes spaghetti and cooks it in his spare time, though many characters seem to think that he's really not very good at it (Frisk describing the taste as "indescribable"). Sans says that ever since he started taking lessons that his culinary talents have improved greatly, and that in a year, his food might even be "edible". He also has a love for puzzles, as shown in his dialogue and the numerous amount of puzzles he prepares whilst the protagonist approaches Snowdin Town.

Main Story
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.

Neutral Route
Papyrus makes his first appearance in Snowdin Forest, where he proclaims to Sans that he will capture a human, unbeknownst to the protagonist hiding behind a conveniently shaped lamp, and berates Sans for not putting in his full effort to prepare his puzzles. Papyrus meets the protagonist for the first time face-to-face further on into the forest, where he is at first distracted by a rock behind the human, but quickly notices the human standing right in front of him.

Papyrus continues to weakly antagonize the protagonist with puzzle after puzzle, but quickly begins to warm up to them, and almost refuses to fight them when they finally meet him alone at the end of Snowdin town. He shakes off the notion, however, and initiates an encounter. Papyrus never actually kills the protagonist, and, if they are reduced to 1 HP, instead captures them and places them within his shed, from which they can easily escape.

Ultimately deciding that this human is friendly, Papyrus lets them continue into Waterfall, giving them advice on how to exit the underground and also offering either a hangout or a date back at their house, depending on whether they insulted or flirted with him, respectively.

Returning to Papyrus' home after this will initiate the hangout or date, where Papyrus will dress in his secret style, and hide a special gift of spaghetti under his hat for the protagonist. Ultimately, however, Papyrus decides that the protagonist cares too much about Papyrus for their own good, and sends them back on their way with his phone number, which can be called at any time later in the game to hear Papyrus' opinions and thoughts on various areas of the underground.

When the protagonist makes their way into Waterfall, Papyrus is seen attempting to sway Undyne from killing them; Undyne refuses to yield, however, and Papyrus finds himself obligated to help her. He contacts the protagonist's phone and asks them to confirm the type of equipment they're currently wearing. Shortly after the call, Undyne finds and ambushes the protagonist. After the protagonist escapes, Papyrus rings them up again and explains that he used the information to try to deceive Undyne. The way in which Papyrus's lie backfired depends on whether the protagonist answered him honestly or changed their clothing afterward.

During the protagonist's ultimate confrontation with Undyne, Papyrus calls yet again and suggests that the three of them hang out; if the protagonist gives Undyne water in Hotland, Papyrus will make good on this suggestion and be waiting at Undyne's home for the player to initiate the hangout. He flees from the premises early on, however, forcing Undyne and the protagonist to become friends one-on-one. After this succeeds, Papyrus invites Undyne to stay at his house (on account of the fire that now lives there).

True Pacifist Route
Papyrus is present later on when Undyne gives you the letter that initiates Alphys's date, and appears finally during the ending of the Pacifist Route, where he is one of the monster friends whose souls Asriel Dreemurr captures, but is ultimately saved along with all monster kind.

In the interactive epilogue, Papyrus discovers that there is no need for the Royal Guard anymore, and proclaims that this is " THE WORST POSSIBLE ENDING ."

In the credits it appears many of Papyrus' dreams have come true: he is shown driving a red car along a highway, something he has always wanted to do, and 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 guard.

Genocide Route
In the 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, describing it as "weird" and rationalizing it as "culture shock". He remains oblivious to the protagonist's actual personality until their battle, where he sees their killing intent and instantly spares them, wishing to guide the protagonist on a better path and be friends.

Should the protagonist choose to kill him, he states he still believes in the protagonist and thinks that they can be a better person. If they choose to spare him, then Papyrus is relieved 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, which acts similar to dating.

In Battle

 * See Papyrus/In Battle

Frisk
Despite wanting to capture you, Papyrus still cares about you, and eventually ends up dating/hanging out with you if you pick the Pacifist Route. He eventually, however, turns you down if you choose to date, as he realizes he does not feel the same way about you.

He does genuinely care about you, though, as shown through keeping you out of danger, and worrying about your well-being when you escape his shed (if he captures you) because you didn't let him know that you were escaping. You remain friends throughout the game.

If you pursue the Genocide Route, in your battle with him, he instantly spares you and wishes to guide to on a better path and be friends. Even if you murder him in this route, he still believes that there is good in you and wants to be friends.

If you allow him to win in battle, he simply captures you and puts you in the shed, and asks you not to escape. Each time you escape (and allow him to win again) this escalates until Papyrus just gives you the option of not being captured at all.

Sans
Sans is Papyrus's brother, and though he often yells at Sans to stop being so lazy and he detests the many skeleton puns dropped throughout the game (especially Sans's), the two of them care about each other a lot. Sans refers to Papyrus as "cool". Papyrus often cleans up after Sans, and states that Sans is lucky to have such a cool guy taking care of him. He was also the one to convince Sans to get a job as a sentry.

Sans cares deeply for Papyrus, a fact made especially evident should the protagonist decide to kill Papyrus. As the protagonist is being judged at the end of the Neutral Route, Sans rationalizes that the protagonist could easily have spared his brother, but chose not to. If the protagonist insists to Sans that they are not his brother's keeper, Sans denounces them as a "dirty brother killer." In a Genocide run, should Sans be slayed by the protagonist, he calls out to the long-dead Papyrus with his dying breath, masking his despair with his usual carefree persona.

Additionally, during an encounter with Shyren, the protagonist can hum with it to start a concert. If Papyrus is alive, Sans will be selling tickets to the concert made of toilet paper. If Papyrus is dead, regardless of a Genocide run or not, instead "a hooded figure watches the commotion from afar." It is heavily implied the figure is Sans.

Undyne
Papyrus and Undyne are good friends. Being the head of the Royal Guard, Undyne took Papyrus under her wing to train him, however instead of giving him fighting lessons, she gave him cooking lessons, because she believed he was too nice to join the royal guard. Still, Papyrus continues to try his best to capture a human so Undyne will let him join the royal guard. He believes that she and the protagonist can truly get along, even offering to be their "wingman" for their friendship.

Flowey
In an optional conversation with Sans, it is implied that Papyrus has encountered Flowey, and that he receives flattery, advice, encouragement and predictions from Flowey, probably to Flowey's benefit. Flowey also told Papyrus to bring him and Undyne, Asgore, Sans, and Alphys, to bid the protagonist farewell. Flowey actually wanted them to gather in order to harvest their Souls to regain his true form.

Flowey mentions in one of his Neutral Route endings that he used his SAVE ability to interact with Papyrus numerous times. He says that Papyrus had started the "Flowey Fan Club" in several different timelines. Flowey says that he found Papyrus as one of the better characters to mess around with and took a long time to get bored of him.

Mettaton
Like most monsters in the Underground, Papyrus idolizes Mettaton as a popular celebrity. He owns several MTT-Brand items, watches his shows, and calls him his " FAVORITE SEXY RECTANGLE " in a call with him in Hotland. Papyrus dubs the ghost who Mettaton once was " HAPPSTABLOOK, THE HAPPY GHOST ", which isn't wrong, it's just his headcanon. Otherwise, the two never officially meet in the main story.

Trivia

 * His name refers to the font Papyrus, which is the font he speaks in as well as a reference to the webcomic Helvetica, which is a webcomic about a skeleton named after a font (acknowledged in the credits of the game, where his "special inspiration" is cited as J.N. Wiedle, the author of the comic).
 * He is the only character in the game whose dialogue doesn't start with an asterisk (*); instead, his dialogue begins without any precursor characters.
 * If you type "Papyru" for the character's name, the response becomes "I'LL ALLOW IT!!! ". This makes Papyrus the only boss character who lets their name be chosen at the beginning of the game (if only because the character limit prevents you from typing out the full name).
 * Following the convention of using one's birth year in their username, this puts him at 14 to 24 years old in the year 201X.
 * He is old enough to drive a car.
 * When you visit Papyrus' house for the first time, right before the date/hangout, continuously walking in and out of Papyrus' kitchen at a rapid pace causes him to dash quickly to the left and to the right (as Papyrus does follow you to the kitchen when you enter in, then goes back to where he originally was as you exit it). You will eventually hear a tinkling sound every time you switch rooms, eventually leading to a different 'completion' sound being heard. If you talk to Papyrus after that, he'll mention he can't feel his legs, which must mean he is a great host.
 * The symbol seen on the right side of Papyrus' armor is nearly identical to the symbol found on the enemy Starman from the game Earthbound. This could possibly be a reference to Toby Fox's Earthbound Halloween ROM Hack, which is also where the song "Megalovania" originated from.
 * Attempting to walk back during the electricity maze will cause Papyrus to tell the protagonist not to be rude, and that he made the puzzle just for them. Backing away again causes Papyrus to ask the protagonist if they treat their mother this way when she makes them puzzles. Doing this again will only cause Papyrus to say " HUMANS... ".
 * Doing this again at the Junior Jumble puzzle will make Papyrus tell Sans that the human is escaping, and that the puzzle is too hard.
 * When encountering Papyrus after a normal reset, he will notice the human looks familiar to him before Sans says they look familiar because of the rock. At the electric maze, Papyrus asks Sans if he knows the human.
 * It would seem his eyes are differently sized, on both his overworld sprite and his battle sprite.
 * According to a tweet by creator Toby Fox, Papyrus is the younger of the skeleton brothers. As if there were a Japanese version of the game, Papyrus would use the term "aniki" (an honorable term for an older brother) when referring to Sans.
 * The Mark on Papyrus' Armour is an exact copy of the one that the Starman from Earthbound has.