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.

Description
She initially appears as an imposing titan in a suit of menacing black and white armor. She later is revealed to be a fish-like humanoid creature with blue scales and a long bright red ponytail. She is incredibly powerful, as shown by her ability to suplex boulders and break rocks just by punching them.

Personality
Undyne 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 in order 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 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
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 rushling of some grass causes Undyne 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 repeatively attacks them, although the protagonist always manages to evade capture either by dodging her attacks and running away, or by the Monster Kid interfering 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. 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 lauches 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.

If the protagonist has previously killed someone, Undyne will state that she refuses to be friends with a murderer.

True Pacifist Route
If the protagonist gives Undyne the water and did not previously kill anyone, 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, 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". She is then determined to become "besties" by way of talking over tea, a fiery cooking lesson that accidentally burns down Undyne's home, and a final battle with neither able to muster any intent to harm each other. During the faux battle sequence you have with Undyne she says that you remind her of someone who she used to train with. This could be a possible reference to Asgore Dreemurr. After the fact, Undyne moves to Papyrus's home, and you can call Papyrus and Undyne at any point in the adventure.

If you have fought Photoshop Flowey, 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. They end up going on a date in the Garbage Dump where they encounter Undyne. When they meet, Alphys talks about her romantic interest in Undyne, and they reconcile emotionally, until Undyne tosses Alphys in the trash. Almost directly after this, Papyrus shows up to jog 100 laps with Alphys "Hooting about how great we are." As this occurs, Undyne questions you about if anime is real or not, in which you get a choice. This said choice does not affect the outcome to the event, and right after it, the event ends.

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.

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 bothered by the Monster Kid, the protagonist attacks them, but Undyne weathers the blow in their stead. She is killed instantly, but thanks to her incredible Determination, she reforms in a new and improved suit of armor as Undyne the Undying, as well as having a light flash out from her formerly covered eye that fades in and out throughout the battle. After being defeated, she declares that she is disappointed in herself for not being able to defeat you, but finds comfort in knowing that Alphys has evacuated the rest of the monsters to safety. She begins melting, shortly before bursting into dust due to her determination.

In Battle

 * See Undyne/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 it in front of the young Frisk. 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 for 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
Undyne has a strong desire to capture the protagonist at first, engaging them in battle and a long chase. Ultimately, in the Neutral Route / 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.

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 neighbourhood. 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
Undyne was aware of Mettaton when he was neighbour 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
Undyne used to give her piano lessons, but one day she stopped coming – probably after her sister fell down.

Trivia

 * If you attempt to name the fallen human "Undyne", the response becomes "Get your OWN name!" and prevents you from using the name.
 * Her name is derived from the Greek water elementals or water nymphs called "Undines".
 * This is ironic, as undines are known as not having souls, while Undyne's soul is one of the strongest in the game due to her high Determination.
 * Her name also doubles as a pun on the words "UNDYing" and "HeroiNE".
 * If you have killed all canine members of the Royal Guard in Snowdin, Undyne will comment on it, saying that the "Snowdin Canine Unit has been completely decimated".
 * If you killed Papyrus, her mood and dialogue before the battle will be completely different.
 * If you let Monster Kid fall in the bridge sequence prior to the battle, her health will be lowered a little before the battle. This can be used as an advantage to those who intend to kill her on a neutral run.
 * In her Neutral/Pacifist Route battle, if the protagonist intentionally fails to block her first volley, she will repeat the same pattern for a total of four turns after which she will start using faster attacks. She explains that she wants to make the fight "fair" by instructing the protagonist on how to block her attacks, but eventually grows irritated as she is not their "freaking kindergarten teacher".
 * Undyne appears to be much calmer in their first encounters in the game, unlike the violent and crude behavior she is in later; she may only act this way as to not harm Monster Kid.
 * If the protagonist calls Undyne while at the "WELCOME TO HOTLAND" sign, Undyne says that she intentionally stopped chasing the protagonist because they had a phone call. In the chase sequence, if the protagonist is far enough from Undyne when they receive the call, she will continue toward the protagonist before abruptly stopping a few feet away.
 * Her improved armor in the Genocide Route bears a very strong resemblance to both Mettaton NEO, Papyrus' Jogboy outfit, and Asriel's final form. This appears to be a common theme among any monster that manages to acquire determination.
 * Many have theorized that her combat style (and Genocide appearance) is based on Dragon Slayer Ornstein, a boss from Dark Souls.
 * When Undyne was younger, she followed Gerson around to watch him beat up bad guys, much like how Monster Kid followed Undyne. However, whenever Undyne tried to help Gerson fight bad guys, she ends up attacking "the mailman or something like that".
 * Throwing away the letter you get from her, then talking to her again will give you "Undyne Letter EX", which cannot be thrown away.
 * Her username on UnderNet, according to a phone call with CoolSkeleton95, is StrongFish91.
 * Following a common username convention of using one's birth year in their username: Assuming Undyne was born in the year 1991, this would put her at 19 to 29 years old during the events of Undertale (201X).
 * According to one of the Neutral Endings, Undyne can bench press seven children. This is probably accurate as she'll suplex large boulders casually if she wants to.
 * On a second run of the game if you've befriended Undyne previously, she'll say that she "remembers" you as a friend and you're a bad person who just manipulates people into liking you.