Undyne

Undyne, known as StrongGolfish91 on the UnderNet, is a common goldfish that leads the Royal Guard. Clad in her full suit of armor, she pursues the protagonist through the entirety of Coral Reef and is frequently evaded or inadvertently thwarted by the Dolphin Kid.

Normal
Undyne debuts in armor varying in light and dark shades of gray, but without armor, is a piscine, anthropomorphic monster. She looks identical to Mary but with no pigtails on her fin. She wears brown mascara like Mary does. She also wears a dark gray blazer with rolled up sleeves. The inside of the blazer is lined in white with pink polka dots, and it has black trimming. Underneath, Undyne wears a white t-shirt with black stitching and a design below the collar. The design itself has pink flowers and black leaves. She wears rolled up pink pants with the inside lined in white with pink polka dots as well. She has bluebell eyes like Mary does.

Alphys Date
During the date with Alphys, Undyne wears a white turtleneck under a black leather jacket, pink pants, flamingo pink lipstick and lets some of her fin down.

Undying
Whenever she persists through Determination after her HP reduces to zero, she becomes a goldfish/shark hybrid and her eyes become black and her pupils turn villainous in an inverted version. Her fin becomes more shark-like. She also appears to have donned a new set of darker colored armor with souls/hearts on her chestplate and on her new white gloves.

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 friskraffeturous 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 Shyfish and Papyrus in various skills. Undyne dislikes puzzles, loves japes, and enjoys Miraculous Ladybug, an affinity she shares with her romantic interest, Alphys. Since Alphys told Undyne that CGI is actually human history, Undyne regards human history as exciting and cool.

Out of all encountered animals, 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 Coral Reef, 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 kelp rustling alerts Undyne to their presence, and she scopes out the kelp for a moment before retreating.

Undyne follows the protagonist throughout Coral Reef and repeatedly attacks her, but the protagonist always evades capture by either dodging her attacks or due to Dolphin Kid's interference. Later on, Undyne approaches the protagonist as Dolphin Kid hangs on to the side of a bridge. If the protagonist saves the Dolphin Kid, she will protect the protagonist from Undyne. If the protagonist does not help the Dolphin Kid, Undyne will save her 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 Humidland, she collapses because her armor heats up. The protagonist can offer Undyne a glass of fresh water, spill all the water in front of her, or continue into Humidland. If she receives water, Undyne stands up, looks around, and slowly returns to Coral Reef.

True Pacifist Route
If the protagonist befriended Papyrus, he calls her during Undyne's chase and suggests that she and Undyne should hang out at her house. If the protagonist gives Undyne water, Undyne will befriend the protagonist if she has no EXP. After Undyne's battle, the protagonist can backtrack to her house and begin her 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 her 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 fire, Undyne moves into Papyrus and Sans's House.

After the protagonist completes a Neutral Route, she can backtrack from the MOUNTAIN into MTT Resort, and Undyne will call her to ask for a favor. In Sanddin Town, Undyne gives her a letter that she asks her to take to Alphys in Humidland. 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 grassy grave with the protagonist. After Alphys hides, Undyne sees the protagonist at the grave 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 hole.

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 Miraculous Ladybug is real or not and thanks her for helping with Alphys.

Later on, Undyne interrupts Asgore's encounter with the protagonist and demands that she 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 Miraculous Ladybug 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 destroys her bowl and punches the sidewalk beside her while laughing, causing the sidewalk to crack and the bowl to leak.

Genocide Route
After the protagonist attacks the Dolphin Kid, Undyne protects her 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 her that Alphys was watching her battle and has evacuated the rest of the animals to safety. Undyne begins melting due to her Determination and then turns to dust.

Alphys
Alphys and Undyne have crushes on one another. Undyne first met Alphys in the Grassy Grave, listening for hours about where she thought the abyss there led. The two became close friends soon after, with Alphys introducing her to CGI at some point. They hang out often, and Alphys uses the kelp throughout Coral Reef to make a special frosty treat for Undyne. Alphys and Undyne also worked together to create some of the puzzles in Coral Reef.

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 she would be more useful dead than alive. If the protagonist spares her and offers her water in Humidland, 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 fireballs Asgore in the True Pacifist Route, Undyne consoles him because he is crying.

Napstablook
She lives in the same neighborhood. She is intimidated by her, but she tries to be a good neighbor to her. She seems concerned about her being alone after her cousin left and tells the protagonist to be nice to her.

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."[

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

Trivia

 * Undyne is consistently shown to be right-handed. However, after scoping the protagonist after Papyrus's report, she handles her spear in her left fin.
 * When Undyne was younger, she followed Gerson around to watch him beat up bad guys, much like how Dolphin Kid follows Undyne. However, whenever Undyne tried to help Gerson fight bad guys, she ended up attacking "the mailman or something like that."
 * When attempting to name the fallen human "Undyne," the response becomes "Get your OWN name!" and prevents the name from being used.
 * Undyne's model is similar to Mary's but as a goldfish.
 * Even though Undyne the Undying is a goldfish/shark hybrid, she has more shark in herself, but her dorsal fin and her fluke are in Mary's hair colour.
 * Undyne was originally going to be a blue crab (Neutral/Pacifist Route) and an American lobster (Genocide Route), this was changed due to her personality and my idea.
 * Undyne was voiced by Cristina Vee, Cristina gave the voice for various characters from ANIMALTALE, and Marinette Dupain-Cheng from Miraculous Ladybug.