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.

Appearance
is a fish-like humanoid undine monster, with blue scales and a long bright red ponytail. She has fins on the side of her head and a pair of protruding teeth. She wears an eyepatch on her left eye. She has yellow sclera; when she's lethally damaged, however, it turns black.

Personality
has an intensely passionate personality. She is 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 that allow for their souls to persist 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 can't handle that much determination.

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

Throughout Waterfall, 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 's attempts to capture the human.

Near the end of Waterfall, 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, will save them instead; this makes her take some HP damage.

After this, 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 '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. Once refreshed, gets back up, pauses for a bit, and promptly leaves back to Waterfall.

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

is initially reluctant to be nice to the protagonist and is hostile, but she is either encouraged 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 '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) who she used to train with. After the fact, 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, calls the protagonist, and asks them for a favor. In Snowdin Town, 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, who decided to give the letter to Alphys herself and is on the search for her, and prompts Alphys to hide. After leaves, Alphys expresses to the protagonist of her dishonesty and feelings toward, and the protagonist suggests they role-play to get Alphys to express her feelings. then walks in during the role-play, where Alphys expresses her lying attitude and her romantic interest. says she admires Alphys's passion and they reconcile emotionally, until 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, questions the protagonist about if anime is real or not (as Alphys had said it wasn't), and then thanks them for helping Alphys before leaving.

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; 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), runs off to truly check on the air-headed skeleton. In the credits, is seen kissing Alphys, who faints.

Genocide Route
After being approached by Monster Kid, the protagonist attacks them, but weathers the blow in their stead. 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 have mutual crushes on each other. 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, can be seen giving Alphys a kiss on her cheek.

Papyrus
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 to capture them. She teaches him how to cook in hopes for a change of mind.

The protagonist
has a strong desire to capture 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. 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 in a previous playthrough, she'll say that she "remembers" them as a friend and they're a bad person who just manipulates people into liking them.

Asgore
sees him as more of a friend than a King. He visits her with some frequency, as 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're intimidated by her (the wrestling invitation didn't 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
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 person. 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 doesn't care for Mettaton as a celebrity and says he gives her bad vibes.

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

Name
"" 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 "", 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 are killed, will comment on it on her pre-battle monologue.
 * Similarly, if Papyrus is killed, the monologue is completely different.
 * When was younger, she followed Gerson around to watch him beat up bad guys, much like how Monster Kid followed . Whenever  tried to help Gerson fight bad guys, she ended up attacking "the mailman or something like that".