Undyne is a weapon-based opponent, wielding a spear to change the protagonist's SOUL mode while using multiple spear patterns and combinations to attack.
Green Mode is used initially, with the red mode being used twice briefly, and then exclusively following the last set of attacks in green mode.
Note: All damages assume no armor, neutral or true pacifist route, and LV 1. In the genocide route, these attacks deal 8-12 damage.
She throws arrows from all sides at the protagonist in Green mode, including specifically slow and fast moving arrows, as well as arrows that flip the orientation that they are approaching from. These attacks do 5 damage per hit.
She restricts the Bullet Board to a minuscule box which launches spears upwards toward the protagonist in three columns. This attack has a pixel-perfect safe spot between the left and center spears. This attack does 5 damage per hit.
She also throws a volley of continuous spears at the protagonist on a larger Board. This attack does 5 damage per hit.
After changing the SOUL mode to red at the end of an attack in green mode, she will throw a single spear at the protagonist. This attack does 4 damage per hit.
She throws rings of spears which enclose on the protagonist while rotating. She has another version of this attack in which the spears do not rotate, but instead head straight towards the protagonist's soul; the spears travel in the direction they were initially facing for a few seconds before fading away. These attacks are Genocide Route exclusive.
To spare Undyne, the protagonist must deflect her spears while in Green Mode and flee from her while in Red Mode (they cannot escape in Green Mode) and run through the cave she was guarding. After completing a number of green phases (first 6, then 4 and then 8), she will switch the protagonist to Red Mode while throwing a spear which can be dodged by moving down. Once in Red Mode, the protagonist can escape. She will chase the protagonist down, and due to her fast speed will catch up with them and switch back to Green Mode until they have endured at least three rounds, each one harder than the last. Undyne the Undying, however, must be fought.
If the protagonist does not escape while in Red Mode, Undyne will throw more spears and then switch them back to Green Mode for the next round. Fleeing from her as soon as possible makes the battle significantly easier than trying to weather the three required rounds of Green Mode in one go, as she progressively increases her projectile speed over the course of the fight while fleeing resets the speed. If the protagonist does not flee, Undyne will continue attacking, and the speed of her attacks will continue to increase. The protagonist can also escape back the way they came from, although Undyne will catch up quickly, as she enters the screen right behind the protagonist.
If the protagonist reaches Hotland, Undyne will stop for a moment to scold Sans for falling asleep, allowing the protagonist to outrun her. The protagonist must then run across a bridge until they reach an area with a water container. Then, Undyne will slowly try to reach them, but she will collapse and faint due to the high heat, and will no longer attack. At this point the protagonist can either keep walking or pour water on her, letting her leave and making the Undyne date available on the Pacifist Route, or a Neutral Route where no monsters have been killed and Papyrus has been befriended.
If Undyne's health reaches 0, she manages to cling on to life with Determination and survives further attacks from the protagonist. The protagonist's SOUL returns to Green Mode as Undyne continues attacking; the arrows move slower each turn. After several turns when this occurs, Undyne begins to ripple, her body starts to split apart. She reforms due to her Determination, causing her to have more Determination than her monster body can handle. Because of this, she melts (similar to the Amalgamates in the True Lab near the end of the True Pacifist Route) and slowly passes away.
Neutral and True Pacifist Routes
Seven. Seven human souls, and King ASGORE will become a god. Six. That's how many we have collected thus far. Understand? Through your seventh and final soul, this world will be transformed. First, however, as is customary for those who make it this far... I shall tell you the tragic tale of our people. It all started, long ago...
[If the protagonist hasn't killed any monsters]
... No, you know what? SCREW IT! WHY SHOULD I TELL THAT STORY WHEN YOU'RE ABOUT TO DIE!?! NGAAAAAHHHHHHHHHHHHHHHHH! YOU! You're standing in the way of everybody's hopes and dreams! Alphys's history books made me think humans were cool... ...with their giant robots and flowery swordswomen. BUT YOU?
You're just a coward!
Hiding behind that kid so you could run away from me again![If the protagonist saved the Monster Kid]
You abandoned your friend just so you could run away![If the protagonist ran away, didn't save, or confronted Undyne while the Monster Kid was in trouble]
And let's not forget your wimpy goody-two-shoes-shtick! Oooh! I'm making such a difference by hugging random strangers! You know what would be more valuable to everyone? IF YOU WERE DEAD!!! That's right, human! Your continued existence is a crime! Your life is all that stands between us and our freedom! Right now, I can feel everyone's hearts pounding together! Everyone's been waiting their whole lives for this moment! But we're not nervous at all. When everyone puts their hearts together, they can't lose! Now, human! Let's end this, right here, right now. I'll show you how determined monsters can be! Step forward when you're ready! Fuhuhuhu![Normal text]
And! And! ... and something's been bothering me, too. The whole time I was chasing after you... I felt something... Strange. The echoes of something warm, something... Something like "I miss being friends with you." BUT THAT'S RIDICULOUS! I WOULD NEVER GET CHUMMY WITH A COWARD LIKE YOU! You! You must have been mind-controlling me! Manipulating people to make them like you... What a Grade-A FREAK! Now enough! For everyone's hopes! For everyone's dreams! And for my own freaking dignity! I, UNDYNE, will knock you down![Alternate text: If the protagonist befriended Undyne on a previous playthrough]
[If the protagonist has killed monsters, but not Papyrus]
...No, you know what? SCREW IT! WHY SHOULD I TELL THAT STORY WHEN YOU'RE ABOUT TO DIE!?! NGAAAAAHHHHHHHHHHHHHHHHH! YOU! You're standing in the way of everybody's hopes and dreams! Alphys's history books made me think humans were compassionate... BUT YOU? You're just a remorseless criminal. You wander through the caverns, attacking anyone in your path. Self-defense? Please. You didn't kill them because you had to. You killed them because it was easy for you. Because it was fun for you.
Do you think it was fun when I found out?
(If the protagonist has killed no significant enemies)
Do you think it was fun when people's family members...
...was dead, because of the whims of a single human?
No. But your time's up, villain! You won't hurt anyone else. A knight in shining armor has appeared. And all the pain you have inflicted on the fallen... Every hope, Every dream you have turned to dust... She's gonna send right back through her spear! NGAHHH! I'll show you how determined monsters truly are. Come on! Step forward and let's end this!
[If Papyrus is dead]
... Forget it. Look. Papyrus didn't come to his meeting today. Say what you want about him. He's weird, he's naive, he's self-absorbed... But Papyrus has NEVER missed a meeting. And no matter what time you call him on the phone... Night, day, afternoon, morning... He ALWAYS answers within the first two rings. ... But now he's gone. And his brother isn't around, either. ... What did you do to him? What did you DO TO HIM? Papyrus, who I have trained every day... Even though I KNOW he's too goofy to ever hurt anyone... ... Go ahead. Prepare however you want. But when you step forward... I will KILL you.
As long as you're GREEN you CAN'T ESCAPE! Unless you learn to face danger head-on... You won't last a SECOND against ME!
When I said face towards danger... I meant face towards the bullets![If the protagonist does not block her attack]
Look. I gave you a spear to block the bullets with. Do I have to explain this any more clearly?[If the protagonist does not block her attack a second time]
WHAT ARE YOU DOING? JUST FACE UPWARDS!!! IT'S NOT HARD![If the protagonist does not block her attack a third time]
I wanted this to be a fair fight. I thought if I beat you like this... It'd truly show how strong monsters can be. BUT NOW??? I DON'T CARE! I'M NOT YOUR FREAKING KINDERGARTEN TEACHER! Unless your kindergarten teacher... DOES THIS![If the protagonist does not block the attack a fourth time]
Not bad! Then how about THIS!?
For years, we've dreamed of a happy ending...
And now, sunlight is just within our reach!
I won't let you snatch it away from us!
NGAHHH! Enough warming up!
Mercy! Ha! I still can't believe YOU want to spare ME![Spare after Undyne re-enables red mode]
But even if I spared YOU...[Spare after Undyne re-enables red mode]
You won't get away from me this time! [Undyne catching up to the protagonist first time]
Honestly, I'm doing you a favor...[Undyne catching up to the protagonist first time]
Heh... You're tough![Neither Sparing nor Fleeing after Undyne re-enables red mode]
But even if you could beat me...[Neither Sparing nor Fleeing after Undyne re-enables red mode]
You've escaped from me for the LAST time! [Undyne catching up to the protagonist second time]
COME BACK HERE, YOU LITTLE PUNK!![Third time and up]
STOP RUNNING AWAY!!![Third time and up]
YOU'RE GETTING IN MY WAY![Low HP]
I WILL NOT BE DEFEATED![Low HP]
[After Undyne's health reaches 0]
You were stronger... than I thought...
So then... ... this is where... ... it ends...
I won't die!
Alphys... Asgore... Papyrus...
Everyone is counting on me to protect them!
In the name of everyone's hopes and dreams...
I WILL DEFEAT YOU!
Come on, is that all you've got!?
You're going to have to try harder than that!
S-see how strong we are when we believe in ourselves?
Had enough yet?
...I won't... ...Give up...
This is what I was afraid of...
This is why I never told you...
I won't die!
I WON'T DIE!
I WON'T DIE!
I WON'T DIE!
(The dialogue gets slower and slower each time she repeats it until it completely stops and the fight ends with Undyne turning into dust.)
The heroine that NEVER gives up.[Check]
Undyne flips her spear impatiently.[Neutral]
Undyne points heroically towards the sky.[Neutral]
Undyne flashes a menacing smile.[Neutral]
Undyne draws her finger across her neck.[Neutral]
Undyne bounces impatiently.[Neutral]
Undyne suplexes a large boulder, just because she can.[Neutral]
Undyne thinks of her friends and pounds the ground with her fists.[Neutral]
Undyne holds her fist in front of her and shakes her head.[Neutral]
Undyne towers threateningly.[Neutral]
Smells like sushi.[Neutral]
Smells like angry fish.[Neutral]
Undyne is hyperventilating.[Neutral, late in the fight]
Undyne is smashing spears on the ground.[Neutral, late in the fight]
Undyne's eye is twitching involuntarily.[Neutral, late in the fight]
Undyne's eyes dart around to see if this is a prank.[Neutral, late in the fight]
You told Undyne you didn't want to fight. But nothing happened.[Plead, failed]
You told Undyne you just want to be friends. She remembers someone... Her attacks became a little less extreme.[Plead, successful]
You tell Undyne her attacks are too easy. The bullets get faster.[Challenge, lowest speed]
You tell Undyne her attacks are too easy. The bullets get unfair.[Challenge, higher speed]
You tell Undyne her attacks are too easy. She doesn't care.[Challenge, highest speed]
Water rushes around you.[Low HP]
Flower pollen drifts in front of you.[Low HP]
The wind is howling...[Low HP]
The spears pause for a moment.[Low HP]
Undyne looks determined. [After Undyne's health reaches 0]
Undyne is smiling as if nothing is wrong.[After Undyne's health reaches 0]
Undyne's body is wavering.[After Undyne's health reaches 0]
Undyne's body is losing its shape.[After Undyne's health reaches 0]
Undyne's body...[After Undyne's health reaches 0]
...[After Undyne's health reaches 0]
[Monster Kid]Undyne... You're... You're hurt...
[Undyne]Hurt? It's nothing.
Next time, listen when I tell you to leave, okay?
[Monster Kid]Undyne... I...
[Undyne]I'll take care of this!
Get out of here!
...heh... "It's nothing"...
No... s-somehow, with just one hit...
D... damn it....
Papyrus... Alphys... ASGORE...
Just like that, I...
I've failed you.
My body... It feels like it's splitting apart.
Like any instant... I'll scatter into a million pieces.
Deep, deep in my soul.
There's a burning feeling I can't describe.
A burning feeling that WON'T let me die.
This isn't just about monsters anymore, is it?
If you get past me, you'll...
You'll destroy them all, won't you?
Monsters... Humans... Everyone...
Everyone's hopes. Everyone's dreams. Vanquished in an instant.
But I WON'T let you do that.
Right now, everyone in the world...
I can feel their hearts beating as one.
And we all have ONE goal.
To defeat YOU.
Human. No, WHATEVER you are.
For the sake of the whole world...
I, UNDYNE, will strike you down!
You're gonna have to try a little harder than THAT.
Damn it... So even THAT power... It wasn't enough...?
If you think I'm gonna give up hope, you're wrong. Cause I've... Got my friends behind me.
Alphys told me that she would watch me fight you...
And if anything went wrong, she would... evacuate everyone.
By now she's called ASGORE and told him to absorb the 6 human SOULs.
And with that power...
This world will live on...!
Heroine reformed by her own DETERMINATION to save Earth.[Check]
The heroine appears.[Encounter]
The wind is howling...[Neutral]
Undyne's dialogue ("You're going to have to try harder than that!") in the Neutral Route after draining all her HP is similar to her final sentence when transforming into Undyne the Undying, just before she fights the protagonist ("You're gonna have to try a little harder than THAT.").
It is the one line of dialogue that is always uttered when Undyne is about to die, implying the Determination within is what makes her taunt the protagonist as she refuses to die or give up.
When Undyne is reformed by her determination, any action taken during this fight makes her dialogue progress, and her body continues to deform until it dissolves into dust.