Undyne

Undyne is an 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.

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

Personality
She is strong-willed and confident as a result of her training under King Asgore. Braggadocios, and unmerciful to her enemies, she over-zealously 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 [Frisk|the player] through the entirety of Waterfall, frequently evaded them, or thwarted by interventions of Monster Kid.

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 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 a letter to the protagonist to be delievered to Alphys after the fight with Mettaton EX. Frisk gives Alphys the letter, which she misinterprets as 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.

Genocide Route
After being bothered by Monster Kid, the player attacks him, but is blocked by Undyne, who takes the blow instead. She is killed instantly, but due to her incredible DETERMINATION she re forms, in a new and improved suit of armor, after being defeated, she declares that she was disappointed in herself for not being able to defeat you, but is encouraged that Alphys has evacuated the towns.

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.

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

Alphys
Alphys and Undyne have mutual crushes on each other. In the True Pacifist Route ending, they nearly kiss, but is interrupted by Toriel, telling them not to do it in front of the young Frisk. 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 and Asriel's final form. This appears to be a common theme among any monster that manages to acquire determination.