Undyne

Undyne (/ˈʌn.daɪn/, UN-dyne) is a fish-like monster that leads the Royal Guard. Clad in her full armor, she pursues the protagonist through the entirety of Waterfall, frequently evaded or inadvertently thwarted by the Monster Kid.

Normal
Undyne is a fish-like humanoid undine monster. She has blue scales and a long bright red ponytail. She appears to not have a nose, she has red and blue fins on the side of her head, and a pair of sharp yellow protruding teeth. She wears bright red eye shadow and has an eyepatch on her left eye. Her eyes have vertical black pupils and a yellow sclera.

Alphys Date
During the date with Alphys, Undyne is shown to be wearing a white turtleneck under a black leather jacket, pants, a light-colored eyepatch and lets some of her hair down.

Undying
Whenever she is brought back by determination whether it be Neutral or Genocide Route, her pupils turn white and her sclera turns black. In Genocide, her Undying form no longer has an eye patch, it's gone, and it reveals her empty eye socket that has a strange light that will continuously shine out of it. Not only that, but her hair will spike at the ends as well as her fins are more rigid and pointy.

Personality
Undyne is badass af. She also has an intensely passionate personality. She often seems to act quickly upon an idea or thought, before she thinks about it or thinks of its consequences. She is also strong-willed and confident as a result of her training under King Asgore and is merciless to her enemies. She over-zealously believes in Asgore Dreemurr and his plan to capture seven human souls to break the barrier and escape the Underground.

She loves to teach, having mentored both Shyren and Papyrus in various skills, as well as instructing the protagonist in how Green Mode works. She has a competitive spirit but is ultimately close with those that she considers friends. She has a deep-seated respect for Asgore and romantic feelings for Alphys, who has warped her perception of human history and culture due to her only exposure through the Eight Humans and anime.

She is the only monster known to possess Determination naturally aside from boss monsters, though in much smaller amounts for their souls that allow persistence after death. She likely only holds a small amount of determination normally, and more is inspired during the Genocide route, as the way her body melts before turning to dust implies that even her body cannot handle that much determination.

Neutral Route
Undyne is first introduced to the protagonist after entering Waterfall, where Undyne appears on top of a ledge. Here, Papyrus attempts to convince Undyne not to harm the human, but to no avail. As the protagonist continues, the rustling of some grass causes Undyne to approach the ledge, and almost throw a spear at the protagonist, although she does not in the end, and vanishes.

Throughout Waterfall, Undyne stalks the protagonist, and repeatedly attacks them, although the protagonist always manages to evade capture either by dodging her attacks and running away or when Monster Kid inadvertently interferes with Undyne's attempts to capture the human.

Near the end of Waterfall, Undyne approaches the protagonist just as the Monster Kid falls off a bridge. If the protagonist saves the Monster Kid, they will protect the protagonist. If the protagonist fails to rescue the Monster Kid, Undyne will save them instead; this makes her take some HP damage.

After this, Undyne confronts the character, and begins a speech, but cuts it short (according to her, because she forgot the words to it), and launches head-on into battle. Here, the protagonist either kills her or flees from her into Hotland.

Once delving into Hotland, the heat of Undyne's armor causes her to collapse. At this point, the protagonist has the choice to bring her some water from a nearby water cooler; there's also the option to spill all the water in front of Undyne. Once refreshed, Undyne gets back up, pauses for a bit, and promptly leaves back to Waterfall.

Should she be killed in battle, Undyne clings to life with Determination and traps the protagonist. However, her determination backfires, eventually causing her to melt and turn into dust.

True Pacifist Route
If the protagonist gives Undyne the water and did not previously kill anyone (as she refuses to be friends with a murderer), the potential to befriend her exists. During Undyne's chase sequence, Papyrus will call the protagonist and suggest they and Undyne should hang out at her house. After Undyne's fight, the protagonist can meet Papyrus at Undyne's house, where Papyrus immediately jumps out the window to leave Undyne and the protagonist alone.

Undyne is initially reluctant to be nice to the protagonist and is hostile. She is then encouraged either by Papyrus's brief return when he says she's "not up to the challenge" or realizes being the protagonist's friend would be her "ultimate revenge," and is then determined to become "besties." By way of talking over tea, she tells the protagonist how she used to train with Asgore and became the leader of the Royal Guard. She then realizes that it is now time for Papyrus's cooking lesson, but since he left, she forces the lesson on the player to further their friendship, which accidentally burns down Undyne's home. Feeling that she had failed, she battles the protagonist once more but realizes neither can muster any intent to harm each other, stating the protagonist reminds her of someone (heavily implied to be Asgore) whom she used to train with. After the fact, Undyne moves to Papyrus's home, telling the protagonist to call Papyrus to talk to her.

After the protagonist has completed the Neutral Route and has not killed anyone, while heading back down from the CORE into MTT Resort, Undyne calls the protagonist and asks them for a favor. In Snowdin Town, Undyne gives them "Undyne's Letter" to the protagonist to be delivered to Alphys. The protagonist gives Alphys the letter, which she mistakenly believes is from the protagonist as the letter lacks a signature. They end up going on a date in the Garbage Dump where they encounter Undyne, who decided to give the letter to Alphys herself and is on the search for her, and prompts Alphys to hide. After Undyne leaves, Alphys expresses to the protagonist of her dishonesty and feelings toward Undyne, and the protagonist suggests they role-play to get Alphys to express her feelings. Undyne then walks in during the role-play, where Alphys expresses her lying attitude and her romantic interest. Undyne says she admires Alphys's passion, and they reconcile emotionally, until Undyne tosses Alphys in the trash. Papyrus then shows up to help Alphys with her lying habits, beginning with jogging 100 laps "Hooting about how great we are." After the two leave, Undyne questions the protagonist about if anime is real or not (as Alphys had said it was not), and then thanks them for helping Alphys before leaving.

Undyne is next seen alongside the other main characters during the confrontation with Asgore, where she demands that Frisk and Asgore cannot fight, as she likes both of them too much. She sees Alphys there as well, and they almost kiss, from the beckoning of Mettaton, but Toriel stops it from happening, as this is all occurring in front of the protagonist. Shortly after the gathering, Flowey ambushes the cast and attempts to kill the protagonist, but his attacks are blocked off by their friends; Undyne encourages the protagonist, saying if they got past her they could get past anything. However, Flowey then proceeds to absorb all the SOULs of the monsters in the Underground.

In the epilogue, after Papyrus runs off and Sans claims to be going to check on him (while walking in the opposite direction), Undyne runs off to truly check on the air-headed skeleton. In the credits, Undyne is seen kissing Alphys, who faints.

Genocide Route
After being approached by Monster Kid, the protagonist attacks them, but Undyne gets in their way and weathers the blow instead. She is killed instantly, but thanks to her incredible Determination and in fighting to protect the monsters of the Underground, she clings to life and reforms in a new and more powerful Undyne the Undying. Despite her increased power, she still falls in defeat to the protagonist, declaring that she is disappointed in herself for not being able to defeat them, but finds comfort in knowing that Alphys has evacuated the rest of the monsters to safety and that they will live on. She begins melting due to her determination, shortly before bursting into dust.

In Battle

 * See /In Battle

Alphys
Alphys and Undyne have mutual crushes on each other. Undyne first met Alphys in the Garbage Dump, listening for hours about where she thought the abyss there led. The two became close friends soon after, with Alphys introducing her to anime at some point. In the True Pacifist Route ending, they nearly kiss but are interrupted by Toriel, telling them not to do so in front of the young protagonist. However, in the credits, Undyne can be seen giving Alphys a kiss on her cheek.

Papyrus
Undyne thinks of Papyrus as a close friend. She is impressed by his persistence and trains him in cooking, but is concerned about his well-being and secretly does not want him to join the Royal Guard because she believes that he is too nice to fight - even befriending the protagonist when he was sent to capture them. She teaches him how to cook in hopes for a change of mind.

The protagonist
Undyne has a strong desire to kill the protagonist, at first, engaging them in battle and a long chase. Ultimately, in the Neutral / True Pacifist Route, arriving in Hotland causes her to faint from the heat combined with the warmth of her armor. This gives the protagonist the option to give her some water.

Later on, Papyrus attempts to make them friends. Undyne resolves to befriend the protagonist out of spite, but transitions to genuine friendship after realizing the protagonist's fundamentally good intentions.

If the protagonist had befriended Undyne in a previous playthrough, she'll say that she "remembers" them as a friend and they are a bad person who just manipulates people into liking them.

Asgore
Undyne sees him as more of a friend than a King. He visits her with some frequency, as Undyne knows his favorite type of tea. Being the hot-headed child she was, she once challenged Asgore to a fight. When she failed to land a single blow on him, Asgore invited her to train with him personally, eventually leading to her current position as head of the Royal Guard. In her earlier years of training, she knocked him to the ground for the first time and felt bad about it, but Asgore simply praised her for her success. She believes that he is the key to saving all monsters.

Napstablook
They live in the same neighborhood. They are intimidated by her (the wrestling invitation did not help), but she tries to be a good neighbor to them. She seems concerned about them being alone after their cousin left, and tells the protagonist to be nice to them.

Mettaton
Undyne was aware of Mettaton when he was a neighbor to Napstablook though she only knew him as Napstablook's cousin and didn't even remember his name. She is not aware that Napstablook's cousin and Mettaton are the same people. She says that "Alphys's friend" once came over to her place and lay on the piano while feeding himself grapes; this was probably Mettaton. She does not care for Mettaton as a celebrity and says he gives her bad vibes.

Shyren
Undyne used to give her piano lessons, but one day she stopped coming – probably after her sister fell down.

Name
"Undyne" is derived from "Undine," a female water nymph from Greek lore. It is also a play on "undying", as Undyne does not die when reduced to zero health before her battle on a Genocide Route.

When attempting to name the fallen human "Undyne", the response becomes "Get your OWN name!" and prevents the name from being used.

Her username on the UnderNet is StrongFish91.

Trivia

 * If either Snowdrake, any of the canine members of the Royal Guard in Snowdin, or Shyren is killed, Undyne will comment on it on her pre-battle monologue.
 * Similarly, if Papyrus is killed, the monologue is completely different.
 * When Undyne was younger, she followed Gerson around to watch him beat up bad guys, much like how Monster Kid followed Undyne. Whenever Undyne tried to help Gerson fight bad guys, she ended up attacking "the mailman or something like that".
 * When Undyne says "En guarde!" when her fight with the protagonist begins, the sentence is misspelled. The right spelling being "en garde", as the sentence is in French. This could also be a portmanteau between the French and English spelling, Garde and Guard, giving Guarde.