Undyne

Undyne, known as StrongFish91 on the UnderNet, is a fish-like monster that leads the Royal Guard. Clad in her full suit of armor, she pursues the protagonist through the entirety of Waterfall and is frequently evaded or inadvertently thwarted by the Monster Kid.

Normal
Undyne debuts in armor varying in light and dark shades of gray, but without armor, is a piscine, anthropomorphic monster. She has blue scales and a long red ponytail. She has red and blue fins on the sides of her head, and a pair of sharp, yellow, protruding teeth. She wears red eye shadow and has an eyepatch on her left eye. She also wears a black tanktop and jeans. Her eyes have black vertical pupils and a yellow sclera. Undyne has no nose.

Alphys Date
During the date with Alphys, Undyne wears a white turtleneck under a black leather jacket, a scarf around her neck, pants, lipstick, a light-colored eye patch, and lets some of her hair down.

Undying
Whenever she persists through Determination after her HP reduces to zero, her pupils turn white, and her sclera turns black. She no longer has an eyepatch, and her newly revealed empty eye socket emits a flashing light in the shape of a spear. Her hair spikes at the ends like a trident and her fin rays have become more rigid and pointy. She also appears to have donned a new set of darker colored armor with souls/hearts on her chestplate and new white gloves, which have spikes protruding from their backside, accompanied by boots with pointed toes.

Personality
Undyne is passionate about everything that she does. She acts quickly on her ideas and is steadfast in her beliefs. Though she wants to defeat the protagonist, she also tries to be fair and offers an explanation of how Green Mode works. Asgore taught her the value of pacifism, which is why Undyne sees the protagonist as "a wimpy loser with a big heart." When the protagonist first encounters her, she fights to further Asgore's plan of collecting seven human SOULs but, if the protagonist shows mercy, Undyne just as quickly defends them from Asgore.

She loves to help others and mentors Shyren and Papyrus in various skills. Undyne dislikes puzzles, loves japes, and enjoys anime, an affinity she shares with her romantic interest, Alphys. Since Alphys told Undyne that anime is actually human history, Undyne regards human history as exciting and cool.

Out of all encountered monsters, Undyne possesses the most Determination. She melts before her death, implying that her body cannot handle the amount of Determination she has.

Neutral Route
The protagonist initially encounters Undyne after entering Waterfall, where Undyne stands on top of a ledge. If alive, Papyrus approaches her and attempts to convince her not to harm the human. As the protagonist continues, the grass rustling alerts Undyne to their presence, and she scopes out the grass for a moment before retreating.

Undyne follows the protagonist throughout Waterfall and repeatedly attacks them, but the protagonist always evades capture by either dodging her attacks or due to Monster Kid's interference. Later on, Undyne approaches the protagonist as Monster Kid hangs on to the side of a bridge. If the protagonist saves the Monster Kid, they will protect the protagonist from Undyne. If the protagonist does not help the Monster Kid, Undyne will save them instead.

After this, Undyne confronts the protagonist and stands atop a crag while giving a monologue that she has practiced many times before. However, she cuts the monologue short because she forgot the rest of it and launches head-on into battle.

After Undyne chases the protagonist into Hotland, she collapses because her armor heats up. The protagonist can offer Undyne a glass of water, spill all the water in front of her, or continue into Hotland. If she receives water, Undyne stands up, looks around, and slowly returns to Waterfall.

True Pacifist Route
If the protagonist befriended Papyrus, he calls them during Undyne's chase and suggests that they and Undyne should hang out at her house. If the protagonist gives Undyne water, Undyne will befriend the protagonist if they have no EXP. After Undyne's battle, the protagonist can backtrack to her house and begin their hangout with her.

Undyne is initially hostile to the protagonist but changes her attitude when Papyrus presents befriending the human as a challenge. She realizes that becoming "besties" with the protagonist would be "the perfect revenge," and says that the protagonist will feel "enamored" with Undyne by the end of their date. Undyne discusses her training with Asgore over tea and suddenly realizes that Papyrus's cooking lesson usually occurs at this time. She insists that her and the protagonist bond over cooking, which ends in Undyne's house catching on fire. Humiliated and feeling that she failed at befriending the protagonist, Undyne battles the protagonist and offers them the first hit. No matter the choice, however, the protagonist will only deal one hitpoint of damage. Undyne won't die and realizes neither of them wants to harm one another.

Because her house is engulfed in flames, Undyne moves into Papyrus and Sans's House.

After the protagonist completes a Neutral Route, they can backtrack from the CORE into MTT Resort, and Undyne will call them to ask for a favor. In Snowdin Town, Undyne gives them a letter that she asks them to take to Alphys in Hotland. The protagonist gives Alphys the letter, but she mistakes it as being a love letter from the protagonist because it has no signature. Alphys reluctantly goes on a date to the garbage dump with the protagonist. After Alphys hides, Undyne sees the protagonist at the dump and asks for her letter back; she wants to give it to Alphys herself. After Undyne walks away, Alphys expresses regret about her lying to Undyne, and the protagonist suggests a roleplay to help Alphys express her feelings. Undyne interrupts the roleplay, and Alphys tells her about her romantic interest. Undyne reciprocates these feelings and tosses Alphys into the trashcan.

Undyne says that Alphys needs to work on her self-image, and Papyrus leaves with Alphys to help with her emotional transformation. After the two leave, Undyne asks the protagonist if anime is real or not and thanks them for helping with Alphys.

Later on, Undyne interrupts Asgore's encounter with the protagonist and demands that they befriend one another. Alphys and Undyne almost kiss before Flowey's ambush, but Toriel chastises them for kissing in front of a child. Flowey ensnares the protagonist's friends with a Photoshop Flowey vine, and Undyne protects them from incoming attacks from Flowey. He then absorbs the six human SOULs and most of the SOULs of the monsters in the Underground.

Epilogue
When the protagonist talks to Undyne in the epilogue, Undyne states that she is glad that she lost to the protagonist and then resolves to watch anime with Alphys. The Royal Guard is disbanded. Toriel also texts the protagonist that Undyne was able to lift Asgore, Sans, Papyrus, Alphys, and Toriel herself at once, but was uncertain as to how to put them down.

On the surface, Undyne pursues Papyrus after he runs off to meet more humans. In the True Pacifist Ending Credits, Undyne kisses Alphys on her left cheek and laughs when she faints in embarrassment. She then punches the sidewalk beside her while laughing, causing it to crack.

Genocide Route
After the protagonist attacks the Monster Kid, Undyne protects them and takes the blow instead. Even though her HP reduces to zero, Undyne holds on through Determination and reforms into Undyne the Undying. If the protagonist defeats her, she tells them that Alphys was watching their battle and has evacuated the rest of the monsters to safety. Undyne begins melting due to her Determination and then turns to dust.

Alphys
Alphys and Undyne love one another. 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. They hang out often, and Alphys uses the seagrass throughout Waterfall to make a special frosty treat for Undyne. Alphys and Undyne also worked together to create some of the puzzles in Waterfall.

They nearly kiss in the True Pacifist Route, but Toriel interrupts them and tells them not to kiss in front of the protagonist.

Papyrus
Undyne 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. She teaches him how to cook in hopes that he changes his mind about joining the Royal Guard. Undyne sometimes enjoys messing with Papyrus, but she often forgets that he usually takes things literally. She is heartbroken and infuriated if the protagonist kills Papyrus during a Neutral/Genocide Route.

The Protagonist
Undyne initially despises the protagonist and believes that they would be more useful dead than alive. If the protagonist spares her and offers her water in Hotland, she begrudgingly befriends them if they have earned no EXP. Though Papyrus tricks Undyne into befriending the protagonist, she genuinely cares about them at the end of their date.

If the protagonist earned no EXP and befriended Undyne in a previous route, she remembers them as a friend but brushes off the feeling before her battle.

Asgore
Even though Asgore is her boss, Undyne sees him as a friend and knows his favorite type of tea. When she was younger, Undyne challenged Asgore to a fight but could not land a single blow on him. She was humiliated because he did not fight back and only dodged her attacks. Asgore then offered to train Undyne and eventually promoted her to head of the Royal Guard.

After Toriel refuses to "be friends again" with Asgore in the True Pacifist Route, Undyne sympathizes with him.

Napstablook
Napstablook and Undyne live in the same neighborhood, and Napstablook is intimidated by her. She still 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 does not know that Mettaton is Napstablook's cousin. She has a distaste for him, and she does not like his show. Though she does acknowledge that she "admires his lifestyle."

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

Trivia

 * "Undyne" derives from "Undine," a womanly water nymph from Greek lore. It is also a play on "undying," as Undyne does not die when reduced to zero health.
 * When attempting to name the fallen human "Undyne," the response becomes "Get your OWN name!" and prevents the name from being used.
 * If either Snowdrake, any or all of the canine members of the Royal Guard in Snowdin or Shyren is killed, Undyne will comment on it on her pre-battle monologue.
 * Papyrus's death supersedes all other pre-battle dialogue and changes her speech the most.
 * Despite donning a helmet during the moment, Undyne's face appears in the dialogue box before her battle as if she had already removed her helmet. If the protagonist approaches to fight Undyne, she appears without her helmet.
 * When Undyne was younger, she followed Gerson around to watch him beat up bad guys, much like how Monster Kid follows Undyne. However, whenever Undyne tried to help Gerson fight bad guys, she ended up attacking "the mailman or something like that."
 * Undyne is consistently shown to be right-handed. However, after scoping the protagonist after Papyrus's report, she handles her spear in her left hand.
 * Despite having an eye patch in her left eye, it is absent in Undyne's west-facing sprites.
 * When Undyne says "En Guarde!" before her fight with the protagonist begins, the sentence is misspelled. The right spelling being "en garde," as the sentence is in French. This could also be a combination of the French and English spelling, Garde, and Guard, giving Guarde.
 * While in Undyne's house for the befriending scene, the table that the protagonist sits at is decorated with a doily. The doily cracks along with the table after Undyne throws a spear at it.
 * Undyne is the only character that recognizes Toriel as Asgore's ex-wife.
 * In Undyne's faux battle, she has one more defense point than in her actual battle, despite being unarmored.
 * Undyne is the only character to have "special" normal attacks. All of her Green Mode spears are either white and blue or yellow but behave the same as white attacks. Also, her spear she throws when doing a green to red switch is white and blue and has a shadow.
 * If the protagonist attempts to kill Undyne on a Neutral Route, they will notice that Undyne's name overlaps the health bar at one point.
 * There's a glitch where if the protagonist is standing as far to the left as possible while initiating Undyne's monologue, they can be shot very high up. The protagonist can then enter the cave, effectively postponing the first fight.
 * If the battle is initiated normally at this point, the protagonist will still be able to move in the air, but Undyne will pursue on a bridge similar to the one heading to the Art Club Room in Hotland.
 * Undyne and Toriel are the only bosses that the protagonist can flee from during a battle on a Neutral Route.
 * However, Undyne's battle can only be fled from if the protagonist's SOUL is red.
 * If Undyne jumps off the cliff to save Monster Kid, her HP is lowered in the following battle.
 * According to Toby, he wanted her armor to be cracked too, but he forgot to add it.
 * Undyne claims to dislike soda, despite owning a bottle herself. However, Alphys owns a cup with some soda in it. It's possible that Undyne keeps it for when Alphys comes over.
 * Undyne tells the protagonist, when they try to interact with the door to her room, that no nerds are allowed. Then, she says that some nerds are, possibly referring to Alphys.
 * In Undyne's faux battle, the protagonist will deal only 1 damage regardless of whether they attack through the FIGHT button or the ACT button.
 * Undyne's presence causes to  in two cases:
 * After the floating tile, before the spear throwing part starts. (the next time the water can be heard, it's back to normal)
 * After the scene where Undyne drags the Monster Kid off screen. (loading a save or re-entering the area reverts this to normal)