Papyrus
Papyrus is one of the main characters in Undertale.
The player is first introduced to Papyrus soon after meeting Sans outside of the Ruins, wherein Sans says they are brothers. How the rest of the encounter, and the player's interactions with Papyrus throughout the game, plays out is affected by the route the player is taking.
In the Genocide route, despite the player not responding to Sans telling them to hide behind the lamp, Papyrus doesn't even acknowledge them standing out in the open as Sans states that he has indeed seen a human, leaving just as quickly as he came. Later on, the player will meet Papyrus again past the Box Road where he's initially talking with Sans about when the human is going to appear. When Sans suggests that he look in the direction your just coming from, Papyrus initially thinks he's talking about the rock behind the player and still doesn't know who, or what, the player is until Sans helps him use the process of elimination. Papyrus will then warn the player, in the playful manner, that they will face many puzzles and risk being captured if they proceed any further. When the player reaches the Invisible Wall Maze puzzle, Papyrus will attempt to explain how it works only for the player to just walk up to him through the puzzle area before it can be activated. Past that area, the player will run into Papyrus and Sans again with "obstacle" of a cross-word puzzle before them. Ignoring the puzzle, Papyrus will point out that it did nothing and Sans will comment that he should have gone with a "Junior Jumble" puzzle instead, which is Papyrus' favorite. Further along, the player will come across several more of Papyrus' puzzles but they will be unactivated. When the player reaches the Colored Tile Puzzle, Papyrus will attempt to explain the puzzle only for the player to once again just walk through it before it can be activated. After talking with Sans, Papyrus will ask the player a question, but they won't respond. To this, Papyrus decides to leave the instructions for the puzzle for the player to look at and do it themselves. When the player reaches the bridge to Snowdin, Papyrus will prepare a more deadly trap for you but will decide not to activate it since the player just walked through all of his puzzles. Finally, when the player is about to leave Snowdin for the Waterfall area, Papyrus will confront them about their behavior including how they walk around with "powder" on their hands. Unaware of what the player has been doing, Papyrus will believe that they are simply starting to lose their way and will offer to be friends so that they will stay on the "straight and narrow." But even after the player kills him, Papyrus will still believe that they can be a good person.
In the Genocide route, if the player spares Papyrus then they won't have to battle Sans at the end. Likewise, if the player takes the Neutral route and kills Papyrus, they won't have to battle Sans but he will make his anger about it clear.
In the Pacifist route, the player will listen to Sans and hide behind the lamp just before Papyrus arrives. Sans will greet Papyrus casually but he'll come straight to the point that Sans hasn't recalibrated his puzzles for days. With Sans' successful use of reverse psychology, Papyrus will ignore the player's hiding place and instead go on about how someone as great as himself will soon get all the things he "deserves" while Sans will never amount to anything. Once Sans starts telling his jokes, Papyrus will storm off saying he has to see to his puzzles. When the player comes out of hiding and is about to leave the area, Sans will ask them to actually show themselves the next time they meet Papyrus, saying that he's been feeling down lately and might cheer up from actually seeing a human. Sans will then assure that Papyrus isn't actually dangerous, "even when he tries to be."
Past the Box Road, Papyrus will be seen talking with Sans and will about to say something about Undyne when they notice the player's approach. Papyrus immediately thinks he has finally found a human, only for Sans to point out that he is actually looking at a rock in the road. Once Sans points out the player in front of the rock, Papyrus will be overjoyed at the prospect of finally getting recognition for his "greatness." He will then proceed to "threaten" the player not to proceed any further before leaving, presumably to prepare his puzzles.
Before meeting Papyrus again, Sans will warn the player of his special "Blue Attack" and that to make it through unharmed you just have to stand still through the attack.
Reaching the Invisible Wall Maze puzzle, the player will find Papyrus accusing Sans of being lazy because he was "napping" all night and referring to Sans calling that "sleeping" as an excuse. Noticing the player, Papyrus will go on to explain how the puzzle works with the player staying in place to listen. The way the puzzle works is that should one touch the invisible walls of the maze a blue orb will unleash a powerful jolt of electricity, but when the player goes to move through the maze Papyrus is the one who gets electrocuted. With Sans suggesting that the player should be the one holding the orb for it to work, Papyrus goes to hand over the orb to you, leaving a distinct trail of footprints showing the way through the maze. Should the player make it through the maze without getting electrocuted, Papyrus will express genuine astonishment at how easily you "solved" the puzzle, only to warn that the next puzzle won't be as easy since it was designed by Sans. Speaking with Sans before moving on from here will reveal that Papyrus' outfit was initially made for a costume party a few weeks earlier, and he thought it was so cool that he hasn't worn anything else since calling it his "battle body."
Arriving at Sans' "puzzle", Papyrus will just start to announce the puzzle only to ask Sans where it was as he didn't see anything noteworthy. The "puzzle" in question is a children's word search left on the ground, and regardless of whether or not the player actually looks at it, Papyrus will quickly point out that it did nothing to stop you. Hearing from Sans that it was a word search, Papyrus will get into a slight argument with him about word scramble puzzles being harder and will ask for the player to settle their "dispute." Talking with Sans before leaving this area will reveal that, the day before, Papyrus got stumped trying to "solve" the horoscope. Right away the player will run into another of Papyrus' traps, which is a plate of spaghetti meant to be so delicious that the player cannot progress. However, the spaghetti is so cold that it froze to the table and the nearby microwave is unplugged, so the player can progress without incident.
After the Dogamy and Dogaressa boss fight, the player will find a series of puzzles set up by Papyrus who has actually gotten himself stuck in place by them and is struggling to solve them again. There are some signs that give hints on what to do for the puzzles, and Papyrus himself is willing to give you hints should you speak with him. Once past them, the player will come to the Colored Tile Puzzle and listen to Papyrus' explanation on how it works. How the player responds to the instructions affects how passage through the puzzle will go. Saying that you don't understand the instructions enough times will result in Papyrus giving up on explaining and just leaving the instructions for the player to read themselves so that they can activate the puzzle once they finally understand. Saying that you understand will result in Papyrus activating the puzzle, only for the tiles to form a perfectly straight and safe path across, with Papyrus leaving in stunned silence. Talking to Sans before leaving this area will reveal that the frozen spaghetti from earlier was actually an improvement from Papyrus' past attempts, and at his current rate, should be able to make something edible within a year.
At the bridge leading to Snowdin, Papyrus will appear to reveal his final puzzle, "The Gauntlet of Deadly Terror." Featuring spears, fire, a giant spiked ball, a cannon and a dog, Papyrus will warn that the player will have only the smallest chance of making it through once he fully activates it. However, Papyrus will hesitate to activate it claiming that he can't use something so direct as it goes against his standards, but his half-hearted laugh at this "decisive victory" would suggest an alternative reason.
When the player goes to leave Snowdin for the Waterfall area, Papyrus will confront them to discuss "complex feelings." Though passing it off as to what the player is feeling, Papyrus states that he's been feeling joy and admiration at finding another who likes pasta and is good at solving puzzles. Just as he's about to declare becoming your friend, however, Papyrus will attack you to carry out his duty. In the battle Papyrus will use his special "Blue Attack", which will turn the player's soul blue and limit their movements by making them fall to the bottom of the battle screen. If the player struggles in the battle, Papyrus can actually "capture" you and lock you up in his shed. However the bars meant to hold you back are wide enough to just walk on through, and Papyrus forgets to lock the door to the shed. To successfully win the battle, the player has to endure Papyrus' attacks with minimal damage and successfully flirt with him. Once he becomes too tired to fight any further, Papyrus will try to dismiss it as you being too weak to defeat him but will quickly fall into despair at how he couldn't stop you, even stating that he will never join the Royal Guard nor will he ever have friends. Becoming friends with Papyrus will not only allow you to pass on through, but he will also give you more specific directions on how to get out of the Underground. Papyrus will then leave for home, giddy with excitement at having a friend.
At this point the player can go on a date with Papyrus, which will help to unlock the True Pacifist route. To start the date, the player has to meet up with Papyrus who will be waiting in front of his house in Snowdin. Deciding to take you someplace special for the date, Papyrus will lead the player a little ways through the town only to return to the house. Inside the player will find various things that give a clearer picture of who Papyrus and Sans are, and of the relationship between the two brothers. On the table right next to the front door, the player finds a rock covered in sprinkles which Papyrus explains is Sans' pet rock that he "forgets to feed" so Papyrus took up the responsibility. Next to the doorway into the kitchen, a dirty sock sits on the floor with sticky notes detailing how Papyrus tried to get Sans to pick it up but eventually gave up. When the player investigates the kitchen sink Papyrus will explain that he increased the size of it to hold more bones underneath, but when the player goes to look, the dog that stole Papyrus' special attack earlier will be found munching on a bone presumably having already eaten all the others. Once you have looked around the house enough, the player can choose to go into Papyrus' room for the rest of the date.
In Papyrus' room the player can find out more about him through his furnishings, including a car shaped bed that he has in place of his dream of driving a car down a highway on the surface. Once the player feels like they have looked around enough, speak with Papyrus to initiate the main date. Admitting that he's never been on a date before, Papyrus pulls out a "official dating rule book" to ensure they have a good time. However, as Papyrus reads through the book, he starts misinterpreting some of the player's actions from earlier on as wanting to date from the start and that you planned it all out. This goes on until Papyrus is convinced that the player is madly in love with him, but will reject their "love" because he doesn't feel any different from when they became friends. Wanting to remain friends, Papyrus will give the player his phone number so they can chat. This will allow the player to talk to Papyrus and hear what he has to say about different areas throughout the game.
At the entrance to the Waterfall area, the player can choose to join Sans for his break. Going to Grillby's for something to eat, Sans will ask the player how they feel about Papyrus, saying that anyone would be cool if they had an outfit like his. Sans will then go on to say that "cool or not" one cannot deny that Papyrus does try his hardest, case in point, the effort he puts into becoming a member of the Royal Guard. According to Sans, Papyrus went to Undyne's house and begged her to let him into the Royal Guard, only for her to shut the door on him because it was the middle of the night. However, after seeing that Papyrus was still waiting there the next day, Undyne decided to give him "warrior" training. At this point, Sans will drown out everything in the bar and ask the player if they know of a "talking flower." After explaining the Echo Flowers to the player, Sans will say that Papyrus has been listening to one particular flower that seems to be doing more than just repeating the last thing it heard; a flower that's been giving him flattery, advice, encouragement and predictions. Sans passes it off as someone else who is playing an elaborate prank on Papyrus, but Sans asks the player to keep an eye out in the Waterfall area.
After the player gets past the first waterfall, they'll see Papyrus approach an armored figure on a ledge just above the path. Addressing the figure as Undyne, Papyrus will report that he fought the human but failed to capture them. When Undyne states that she will take the human's soul herself, Papyrus will try to talk her out of it but will quickly be told to carry out his orders. Throughout the rest of the Waterfall area Papyrus will call you a few times on your cell phone. The first time will be to find out what you are wearing, saying that he's "asking for a friend" who thought that she saw you. The next time Papyrus calls, he will tell you that he told his friend your response to his question "knowing that you would change your outfit after such a question." The third and final time Papyrus will call will be while the player is trying to reach Hotland while running away from Undyne, where he will suggest that you meet up with him at Undyne's house so that you can become friends with her. Following through on this is how the player can befriend Undyne.
Returning to the Waterfall area and going to Napstablook's neighborhood, the player will find Papyrus in front of Undyne's fish-themed house. Agreeing to follow along with his plan to make you and Undyne "best friends", Papyrus will ask you to stand behind him and tell you to give Undyne a bone that he brought along. When Undyne answers the door, Papyrus will say that he is ready for his training and that he had brought along a friend, revealing the player to Undyne. Undyne's expression will initially turn hostile once she recognizes you, but seeing Papyrus' smiling face will prompt her to invite you into the house. Once inside, Papyrus will give Undyne the bone saying that the player brought it on their own as a gift. After putting it away with a bunch of other bones, Undyne will ask if everyone is ready for the training session only for Papyrus to say that he has to go to the bathroom. He will then proceed to jump out a nearby window, leaving the player alone with Undyne. Should the player take the choices that will make Undyne say that she'll never be friends with you, Papyrus will peek in through the broken window saying that he thought Undyne would be "up to the challenge" of befriending you but says aloud that he overestimated her. This will convince Undyne to become friends with the player and she will treat them with, slightly aggressive, hospitality. As the two of you talk over tea, Undyne will soon bring up the subject of her training Papyrus. The truth is, while she does recognize Papyrus' strength and loyalty, Undyne considers him too nice and naive to become part of the Royal Guard. Case in point, Papyrus was supposed to capture you, the last human needed to have the power to destroy the Barrier, and instead became friends with you. Which was why instead of combat training, Undyne was giving Papyrus cooking lessons so that he can do something else with his life. From this point onward, Papyrus will only "appear" when you call him on his cellphone, or when you go to his house in Snowdin.
In the Neutral Pacifist ending, Sans will be telling the player about what has been going on since they left when Papyrus will come in demanding to know who he is talking to. Even though Sans says that he's talking to nobody, Papyrus will want to talk to this "nobody" and will immediately recognize the number as yours. Right away, Papyrus will tell the player that he is now Captain of the Royal Guard and that it is everything he had dreamed of, except that his duties include waters flowers and helping Dr. Alphys with her research rather than fighting. Just as Papyrus starts explaining how Undyne's been doing, she will come in and take the phone, with Papyrus urging her to not "noogie the phone." Undyne will then specify that since Toriel declared that humans will not be treated as enemies, the Royal Guard disbanded leaving Papyrus as the only member and now, technically, her commander. Just when Undyne tries to call Toriel over to join in the call, Papyrus will note that they may not get the phone back for a few hours if she knew who the caller was and decides to spare the player of that.
In the True Pacifist route, when Undyne calls the player to ask something of them, she'll make a remark indicating that Papyrus was the one who gave Undyne the idea of asking Alphys out on a date via a letter. During the date, when Undyne and Alphys are talking about Alphys' situation, Papyrus will appear and declare that he shall be Alphys' "confidence coach."
After the player traverses the True Lab and returns to King Asgore, Papyrus will be the third of your friends to arrive to stop Asgore from fighting you. Seeing Toriel, Papyrus will ask the player if Asgore cloned himself and had his cloned shaved. When Sans shows up, Papyrus will initially be overjoyed when Toriel says that she's heard great things about him from Sans, but will quickly declare this "the worst day of my life" when Toriel reveals that she to loves jokes. After Toriel states that the player would do just fine living in the Underground with all their friends, Alphys will point out that it was Papyrus who called just about everyone and told them to all come here. When she asked how he knew where to go, Papyrus will say that "a little flower told me." Just as Alphys processes what Papyrus meant, the flower in question makes their appearance and traps everyone. When Flowey tries to claim the player's soul for more power, Papyrus and the others will use some of their power to protect you until Flowey absorbs their souls and the souls of every monster in the Underground, turning back into Asriel. During the battle against Asriel, he will taunt the player by saying that every time you "die" during the battle everyone's memories slowly fade away and they will eventually forget you altogether. When the player fails to recover their SAVE file, they decide to use that power to instead SAVE their friends. When the player goes to SAVE Papyrus and Sans, Papyrus will speak of his original intent of capturing a human to become popular but as the player takes actions they have done with him before, such as asking for help with a puzzle or asking for something to eat, Papyrus will regain all of his memories.
After the Barrier is destroyed and Chara is fully purged from the player, Papyrus will be among those who will wait to go up to the surface with Frisk. Once back on the surface, Papyrus will ask Sans what the "giant ball" in the sky is and will be amazed at finding out that it is the Sun. During the credits, Papyrus will be seen finally living his dream of driving a car down a highway, only for Sans to speed right past him on a bicycle.