Undyne

Undyne is a Monster that the player meets in Waterfall. She is the captain of the Royal Guard and lives in Waterfall. She is the third boss in the game.

Appearance
She initially appears as an imposing titan in a suit of menacing black and white armor. She later is revealed to be a fish-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 she is unmerciful to her enemies. She overzealously believes in Asgore Dreemurr and his plan to capture seven 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 player 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.

Main Story
Undyne is first met after leaving Snowdin before Waterfall. Clad in her full armor, she pursues the protagonist through the entirety of Waterfall, frequently evaded or thwarted by the Monster Kid.

Neutral / True Pacifist Route
Eventually, after Monster Kid decides to return home, Undyne confronts the character head-on in battle, where she is either killed or fled from until she becomes exhausted in Hotland. The player at this point has the choice to bring her some water or not.

If they aided Undyne (and did not kill anyone) the potential exists to befriend her later in the game by way of Papyrus and his encouragement, eventually becoming "besties" by way of talking over tea, a fiery cooking lesson, and a final battle. After the fact, Undyne can be found at Papyrus's home, due to hers having been burnt down from the intense cooking session.

After becoming "besties," Undyne gives "Undyne's Letter" to the protagonist to be delivered to Alphys after the fight with Mettaton EX. Frisk gives Alphys the letter, which she mistakenly believes is from Frisk. 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.

Undyne is next seen immediately before the final boss, 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 Frisk.

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 player 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 spear through her 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.

In Battle
She is a weapon-based opponent, mainly dealing with spears in combat.

Neutral / True Pacifist Route
She throws arrows from all sides at the player in Green Mode, including specifically slow and fast moving arrows, as well as arrows that flip the orientation that they are approaching from. She also launches spears in straight lines at the player in Red Mode. To spare Undyne, one must flee from her while in Red Mode and run through the cave she was guarding. She'll chase you down, so you may have to survive 3 volleys, each one harder than the last. Fleeing from her makes the battle significantly easier than trying to weather all of her volleys in one go, as she progressively increases her projectile speed over the course of the battle, but every time you flee, the speed resets. After you complete 3 green phases, she will begin each new battle in Red Mode allowing you to immediately escape. Note: You cannot flee while in Green Mode. You can also escape back the way you came from, although it takes 2 battles to do so. Doing this will allow you to heal using a save point before doing her 3rd volley.

Genocide Route
In Red mode In Green Mode, she can conduct the same battle method as in the Neutral or True Pacifist route, only harder.
 * She can restrict the Bullet Board to a very small box which launches spears upwards toward the player in three columns.
 * She also throws a volley of continuous spears at the player on a larger Board
 * She can throw rings of Spears which enclose on the player while rotating

When mortally wounded by the player, Undyne manages to hang on through Determination, but eventually melts. (In a Genocide run, the lethal wound is received before the battle even begins.) This is consistent with Alphys' experiments in injecting Determination into monsters, although Undyne is the only known case of a monster with innate Determination.

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 player when he was to capture them. She teaches him how to cook in hopes for a change of mind.

The Player
Undyne has a strong desire to capture the player 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 player the option to give her some water.

Later on, Papyrus attempts to make them friends. Undyne resolves to befriend the player out of spite, but transitions to genuine friendship after realizing the player'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 neighbour to them. She seems concerned about them being alone after their cousin left, and tells the player 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

 * Her improved armor in the Genocide Route bears a very strong resemblance to both Mettaton NEO, Papyrus's 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 Dragonslayer Ornstein, a boss from Dark Souls.
 * Her name may be derived from the Greek water elementals or water nymphs called "Undine".
 * Her name is a pun on the word "Undying".
 * 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".
 * If you type "Undyne" for the character's name, the response becomes "Get your OWN name!" and prevents you from using the name.
 * Throwing away the letter you get from her, then talking to her again will give you "Undyne Letter EX", which you cannot read or throw away.
 * She is StrongFish91 on a social network.
 * According to one of the Neutral Ends, 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.
 * 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.