Undyne
Undyne is one of the main characters in Undertale.
The player first hears about Undyne during their second encounter with Papyrus, where the player comes in just as Papyrus is about to say something about her but then notices you. Undyne will make an actual appearance when the player enters the Waterfall area past the first waterfall. When the player enters the patch of tall grass, Undyne will be seen on the ledge above the path. If the player spared Papyrus earlier on, he will appear to report to Undyne that he fought the human but failed to capture them. When Undyne states that she'll take the human's soul herself, Papyrus will attempt to discourage her only to be promptly commanded to carryout his orders. Undyne will then appear throughout various points in the Waterfall area to attack the player, with one of the Monster kids from Snowdin following you along in the hopes of seeing Undyne fight. When you reach the end of the Waterfall area, the player will have a final confrontation with Undyne that will play out differently depending on the route the player has taken.
If the player is taking the Genocide route, the Monster Kid will confront the player about their actions and proceed to stand in their way. When the player attacks, Undyne will appear at the last moment and take the blow, saving the kid. But just when it seems like Undyne will turn to dust, she will slowly start to regain her form saying that she feels a burning feeling within her soul that won't let her die. Realizing that the threat is far greater than she thought, Undyne will feel the hearts of Monsters and Humans alike beating as one with the common goal of stopping the threat that the player has become. She will then transform into Undyne the Undying, the heroine of the Genocide route's story, and fight the player. Whether the player actually fights back or not doesn't matter too much, for, ironically, the Determination that transformed Undyne will slowly kill her. With her dying breath, Undyne will tell the player that Alphys was watching the fight and by now will have begun evacuating the Monsters to safety, and that once Asgore absorbs the six human souls he'll have the power to ensure that the world will live on.
If the player is taking the Neutral route and they have killed Papyrus, they will meet Undyne at the entrance leading to Hotland. Just as she's about to recite the history of the Monster Kingdom, she will stop herself and instead point out that Papyrus didn't come to his meeting that day nor has he been answering his phone. She will demand to know what the player has done to Papyrus, and make it clear that if they try to proceed, she will kill them. When it seems that Undyne is defeated, her determination will keep her alive a little longer and will trying even harder to fight back. But as the player continues to attack, Undyne's body will falter more and her attacks will weaken even as she remains determined to stop the player. Before she completely turns to dust, Undyne's body will reform momentarily and will give a final cry of "I WON'T..."
If the player is taking the Pacifist route, Undyne will confront them at the entrance to Hotland and will start to recite the Monster Kingdom's history. Undyne will then stop herself and mock the player for how they have been befriending the residents of the Underground, when Asgore needs just one more human soul in order to have the power to destroy the Barrier. During the battle, Undyne will elaborate on how the player's efforts are in vain by stating that no human has ever beaten Asgore and that killing you is basically an act of mercy. As the player continues to flee from Undyne, they will eventually reach Hotland where the sweltering heat combined with Undyne's heavy armor will cause her to faint. Using a nearby water cooler, the player can save Undyne by splashing a cup of water on her, allowing the player to later on befriend Undyne.
To befriend Undyne, the player has to return to the Waterfall area where Napstablook's house can be found. Go to the left behind the Save Point, there you will find a fish-themed house where Undyne resides. For the event, Papyrus will be standing outside of the house waiting for you. He will tell you that he's got a plan to make you and Undyne best friends, which involves bringing you along for what is supposed to be his "one-on-one" training session with Undyne. Inside the house just when Undyne asks if everyone is ready to start the training session, Papyrus will say that he has to use the bathroom and proceed to jump out the window, leaving the player and Undyne alone. Undyne will then ask if you came over just to humiliate her further. There are several ways to respond to this and her other questions, but they will ultimately lead to Undyne deciding to get "revenge" by becoming your friend. She will then ask you to take a seat and insist, rather violently, to let her prepare you some tea letting you use her spear to choose which kind you would like. Once you two sit down for tea, Undyne will remark how the tea the player chose is also Asgore's favorite kind and, thinking about it, that the player is very similar to Asgore overall. She will then go on to tell the player that she once tried to prove her strength by attacking Asgore, only for her every attack to miss and Asgore never once fighting back. She was surprised when, after finally stopping her attacks, Asgore apologized to her and asked if she would like to learn how to beat him. Immediately agreeing, Undyne began her training and eventually became head of the Royal Guard. She will then explain that her experience training under Asgore, and in turn training others, is partly why she doesn't think she could ever let Papyrus into the Royal Guard. While Undyne does recognize his strength she finds Papyrus too innocent and nice to send into battle, pointing out the fact that he was supposed to capture the player but ended up befriending them instead. So instead of teaching Papyrus how to fight, Undyne's been "training" him on how to cook. At this point, Undyne will realize that she was supposed to be giving Papyrus his cooking lesson and decide to have the player take the lesson instead, thus becoming closer as "best friends." Unfortunately, this only results in Undyne laying waste to her own house and thinking that she failed at becoming good friends with the player. She'll then engage the player in battle in the hopes of finally settling things, but if the player chooses to fake attack her she'll realize she doesn't have the heart to hurt them because they remind her too much of Asgore. Declaring that she'll just wait for a "mean" human to drop in to the Underground, Undyne and the player will leave the blazing house with Undyne deciding to go "hang out" at Papyrus' home in the meantime.
Later on, at Dr. Alphys' laboratory, the player finds out during Mettaton's Quiz Show that Alphys has a huge crush on Undyne with Mettaton stating that Alphys writes Undyne's name in the margin of her notes, names programing variables after Undyne and even writes stories of the two of them living together.
In the ending for the Neutral Route, when Sans calls, the player finds out that in the time since leaving the Underground that Undyne has handed over the position as head of the Royal Guard over to Papyrus and has been helping out Dr. Alphys with finding an alternative way out of the Underground. At that point, Undyne will take the phone and specify that Papyrus is in fact the only member of the Royal Guard now, since Toriel declared that all humans who fall into the Underground will be treated as friends. Further more, as well as being Alphys' lab assistant, Undyne also works as a gym teacher at a new school Toriel opened. However, when Undyne starts talking about Alphys, the way she speaks hints that Alphys has not admitted her feelings to Undyne and has become even more reclusive.
To fulfill the True Pacifist route, the player will have to Continue the game from their Save before Asgore's boss fight, and fulfill a request from Undyne. To recieve the request, the player will have to leave the New Home and go to the entrance of The Core. There, the player will recieve a call from Undyne saying that she has something for you to deliver. Arriving at Snowdin to meet her at Papyrus' house, Undyne will give the player a letter that's addressed to Alphys, saying that she can't deliver it herself because she can't stand Hotland's temperatures and that she will kill you if you read the letter.
When the player delivers the letter to Alphys, it turns out that the letter was a means of asking Alphys out on a date but Undyne had neglected to sign her name, so Alphys initially thought that it was written by the player. Going to the dump for the date, the player and Alphys only just arrive when Alphys sees Undyne coming. She'll immediately hide behind a nearby trashcan just before Undyne arrives, asking the player for the letter so she can deliver it herself. Once Undyne leaves, Alphys explains that while she does really like Undyne she's scared that she will hate her once she knows the truth. When Alphys and the player rehearse a mock-date to prepare her, Undyne overhears Alphys shouting and thinks the two are on an actual date at first. After some stammering, Alphys finally admits most of the truth about herself to Undyne mainly about how her "research" was really just watching anime and reading comics. In response, Undyne says that while she isn't into "nerdy" things that much, she cares more about how passionate Alphys is for it all and is willing to train her to become confidant enough to not have to lie about herself to others. At this point, Papyrus steps in to do Alphys' training, leaving the player with Undyne who is now concerned about how "real" Alphys' anime and comics actually are.
After the player traverses the True Lab and returns to the Barrier where Asgore awaits, Undyne will be the second of your friends to arrive to stop Asgore from fighting you. Noticing Toriel, Undyne immediately comes to the correct conclusion that she is Asgore's "ex" despite the fact that this apparently their first meeting. When Asgore starts feeling down from seeing how well Toriel gets along with Sans, Undyne tries to cheer him up only for Mettaton to come along and say that she and Alphys should kiss. When Flowey appeared to claim the player's soul for more power, Undyne and the others were able to use some of their power to protect you until Flowey absorbed 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 Undyne, she will continually speak of her original intent of killing off all of the humans but as the player takes actions that they've done to her before, such as fake hitting her and talk about cooking, Undyne will regain all of her memories.
After the Barrier is destroyed and Chara is fully purged from the player, Undyne will be among those who will await to go up to the surface with Frisk. Talking with her before going to check on the rest of the Underground will allow Frisk to tell Napstablook that Undyne says "hi." Once back on the surface, Undyne will remark at how the sunlight and fresh air feel so good, and during the credits, she will be seen at the beach with Alphys giving her a kiss that knocks her unconcious.