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. She is the best, and only fish in town.

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-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. Long time ago she was a pirate in the seas, but one day a harpoon strike her in the eye.

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 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.

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. Undyne fighting style is pirate like and still sometimes acts like one during the battle.

Neutral / 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 the 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 Monster Kid, the player attacks him, but Undyne weathers the blow in his 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. 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 / 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. 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 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. 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. Alphys also opened up Undyne to the human world through Anime.

Papyrus
Undyne thinks of Papyrus as a close friend. She trains him in both combat and 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. In the Neutral / Pacifist Route, arriving in Hotland causes her to faint from dehydration. This gives the player the option to give her some water. Later on, Papyrus attempts to make them friends, ending in achievement.

Asgore
Undyne sees him as more of a friend than a King. She cares for him as evidenced by the fact that she prepares his favorite tea whenever he visits. Being the hot-headed child she was, she challenged Asgore, resulting in being taken under his wing. In her earlier years of training, she knocked him to the ground for the first time and felt bad about it. She believes that he is the key to saving all Monsters.

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.
 * 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. She is still a pirate in spirt and when she is a royal guard she is a pirate.