Mad Mew Mew appears in one of 3 dresses when first encountered. She will appear in the same dress during the fight she wore when revisiting her in front of Undyne's house in the epilogue of the True Pacifist Route. In battle, Mad Mew Mew has a humanlike body with a cat tail. She wears a frilly tutu and ballet shoes. Her head is able to come off when the player attacks her, which she catches in her hand and puts it back where it was. She also wears a t-shirt.
Just like when she was Mad Dummy, she is constantly angry and a bit rambunctious.
After the fight against Mad Dummy, its ghost possesses a Mew Mew doll and unsuccessfully attempts to fuse with it. According to them, the doll was found in a "sciencey place", which heavily implies that the doll belonged to Alphys, a fan of Mew Mew Kissy Cutie.
When the protagonist accesses the Dog Shrine by entering the sink of the kitchen, they will find it abandoned and destroyed with the Annoying Dog on the middle of the wall replaced by a door. The door has a mysterious seal on it and colors corresponding with the Nintendo Switch's Joy-Con. This seal will fall off after defeating Mettaton EX and can be opened by moving both analog sticks apart from each other. Inside is a small room with a Mew Mew doll in the center. Interacting with it and then either attacking or touching the doll multiple times will initiate the fight.
True Pacifist Route
The same thing happens as it does in the Neutral Route, except after the fight with Mad Mew Mew, the training dummy in front of Undyne's house will disappear (regardless of whether the protagonist chose to fight or spare Mad Mew Mew) until the True Pacifist ending is achieved, where Mad Mew Mew appears in front of Undyne's house. During the True Pacifist Ending Credits, Mad Dummy's character credits have been replaced with Mad Mew Mew's.
The SOUL is split into two halves, respectively controlled by both sides of the Joy-Con. Each half of the SOUL can move individually. The SOUL halves are restricted to three positions inside the Bullet Board, closely resembling Muffet's fight.
Mad Mew Mew's attacks scroll vertically from the top of the screen. Eventually, she fires circles with one half being gray and one half being either red or blue as well as the regular white attack. The red/blue half of the circle only allows the red/blue part of the SOUL to pass through.
Mad Mew Mew targets the middle point inside the bullet board to project three cat-faced bullets from her magic wand.
After a while, she turns the bullet board into a plus sign (bordered with an octagon) that has five positions to occupy instead of three and starts attacking horizontally as well as vertically.
If the protagonist manages not to get hit once during the fight, Mad Mew Mew will call them a freak for being able to dodge everything and perform a secret attack which is faster and harder to dodge.
Mad Mew Mew does not take damage if she is hit during the battle. However, her head comes off, letting the protagonist see extra dialogue. This does not affect the True Pacifist Route in any way. She eventually stops attacking once she realizes that fighting the protagonist isn't helping her fuse with her body. This gives the protagonist a chance to talk to her using the ACT menu. If the protagonist tells her that anger might not be the answer, she spares them, asking to "show her what LOVE is." At this point, it is possible to either kill or spare her.
- ENOUGH!!! ENOUGH!!!! ENOUGH!!!!!! | FOOL!! DON'T YOU KNOW NOT TO TOUCH PEOPLE YOU DON'T KNOW!? | ESPECIALLY!! WHEN!!! THEY'RE!!! | Ticklish. | Huh? You don't recognize me...? | Ohhh ho ho… | That's all for the better~! | Once I fuse with this PERFECT new body, mew~ | A brand new, WONDERFUL life is going to begin! | ... but as much as I try to fuse… | IT'S!! NOT!!! WORKING!!!! | MY EMOTIONS AREN'T PURE ENOUGH---!! | I need TENSION! CONFLICT! | Something to bring out my ANGER!!! | ... That's it!!! | YOU!!! Human!!! | Fight with me!!! | COMBAT!! BLOODLUST!! DESTRUCTION!! | It's the perfect emotional cocktail… | To fuse with my new body, mew~ | And, with my new special ability… | This battle will be TRULY INTERESTING!!!
- What are YOU looking at!? [Refight]
- FOOL! You think you can hurt ME with that puny weapon!? | As long as my guard's up, this body is INVINCIBLE! | (And cute, mew~)[Attack]
- WERE YOU LISTENING!? You can't hurt me!!! | Wait. Wait! WAIT! | I get it!! You're trying to bring out my deepest feelings! | Hack away, you despicable fleshling! [Attack#2]
- Wondering where I got this body? | WOULDN'T YOU LIKE TO KNOW!? HA HA HA!!! | Sure, I'll tell you.
- I found it in... A strange place. | A... sciencey place.
- Don't you think I could EASILY become the world's CUTEST dullahan!? | I just have to find a giant cat for my steed, and I'll off! | No idea what was going on THERE, but... | As soon as I saw this body, I just knew －－ | IT WAS ME!!! ME!!! ME!!!!
- So I TOOK it! You can't blame me~ | Why does a scientist need a life-sized girl statue anyway!?
- I probably saved it from EXPERIMENTS!!! | I'm a HERO, and I rescued MYSELF!!!
- ALRIGHT, it's starting to get ANNOYING again!!! | And now, with this new body, | I've got a BRAND NEW life ahead of me!!
- NEW LIFE!!! COOL LIFE!!! NEW LIFE!!! COOL LIFE!!! NEW LIFE!!!
- And NOW, I'll finally be able to... | I'll... finally be able to... | Uhh, continue being Undyne's Training Dummy. | BUT BETTER!!!
- Ah, Undyne... Sweet, violent, Undyne~ | I can't wait to see the look on her face when she sees me~
- She'll blast me with a storm of spears~ | I'll be the world's greatest pin cushion~
- H-Huh...!? That look on your face －－ | She's already got... | ... somebody else?
- ... | Heheheh... Heheh... | Well, if she's got somebody else... | THEN YOU KNOW WHAT I'M GONNA DO TO THEM!??
- I'll RESPECT THEM!!! I'll RESPECT THEIR RELATIONSHIP!!! HAHAHA!!! HAPPY FOR HER!! NOTHING BUT THE BEST!!! BEST OF LUCK!!!
- Enough. ENOUGH. ENOUGH!!! | WHY haven't I FUSED yet!? | Do I have to use my FULL POWER!?
- COME ON! IS THIS ENOUGH FOR YOU!?
- If the protagonist didn't get hit in the battle
- What the hey!? You're some kind of freak, aren't you! | You dodged ALL of my attacks without getting hit! | Unbelievable. Unbelievable! UNBELIEVABLE!!! | YOU! Let's see what you think of THIS, then!!
- EAT THIS!!! AND THIS!!! AND SOME OF THIS!!! BUT NOT THIS!!! DON'T EAT THAT!!! IT'S FOR ME!!! I DIDN'T MEAN TO USE IT!!!
- Huuuuu...!!! | Don't think that was NEARLY enough to earn my praise!!! | Take THIS!!!
- Mewsers! That was Gnarly! | Tubular! Way Cool! Awesome! Groovy! Mondo! Outrageous! Funky! | IS THAT WHAT YOU WANTED ME TO SAY!? BECAUSE WE AREN'T DONE YET!!
- Enough. ENOUGH. ENOUGH!!! | WHY ISN'T IT WORKING!? | NO MATTER HOW MAD I GET, | NO MATTER HOW MUCH I FIGHT, | I JUST CAN'T FUSE WITH THIS BODY!!! | WHAT AM I DOING WRONG!?!? | What am I doing wrong...
- READ THE ROOM!!! | What am I doing wrong...? [Attack]
- H... huh? | You mean I might have to try... | ... other... ...emotions? | You mean something like... | Love? | Then, I... | ... | Human!!! Go ahead!!! | Show me what LOVE is!!! [Talk]
- Show me what LOVE is!!! [Talk, after Mad Mew Mew can be spared]
- I suppose I should have expected that. [Attack, after Mad Mew Mew can be spared]
- Post-Battle (After sparing Mad Mew Mew)
- Such... such kindness...!
- It's... making me FEEL something!
- It may be a while before I can truly fuse with this body.
- But, I think this is a start...!
- Heh, to think that even though I hid here to be alone...
- ... I somehow made a friend!!!
- Thank you.
- Thank you!
- THANK YOU!!
- Post-Battle (After attacking Mad Mew Mew)
- (The Animatronic Limited Edition Mew Mew Machine was shattered into pieces...)
- YOU! I had finally found the perfect body...!!!
- I even came to a secluded place to fuse in PEACE, but...
- I guess I made an ENEMY!!!
- Just WAIT! After I repair this body, you're REALLY going to suffer!!!
- There's a doll here. [Neutral]
- It's a life-sized Mew Mew Doll. For... something? [Check, pre-battle]
- Mew Mew 2: Mew Mew's Pissed Off And Ready To Mew! [Check]
- (...?) [Fight]
- (... it sounded like the doll was... laughing...?) [Fight #2]
- You scratched the Mew Mew Doll gently behind the ears. | (...?) [Touch]
- You scratched the Mew Mew Doll gently behind the ears. | (... it sounded like the doll was... laughing...?) [Touch #2]
- You scratched the Mew Mew Doll gently behind the ears. [Touch#3]
- Smells like perfume and nerds. [Neutral]
- It glares at you. [Neutral]
- Mad Mew Mew tightens her head. [Neutral]
- Mad Mew Mew glares into an elaborate pink mirror, then turns to you the same way. [Neutral]
- Mad Mew Mew considers ska dancing but the music isn't nearly appropriate enough. [Neutral]
- A pre-recorded, peppy voice squawks out of Mad Mew Mew's torso. [Neutral]
- Mad Mew Mew is mewing madly. [Neutral]
- You talked to Mad Mew Mew. | ... doesn't seem much for conversation. [Talk]
- It seems like Mad Mew Mew might listen to you now... [Neutral]
- You told Mad Mew Mew that anger might not be the only way. [Talk]
- ... but it was hard to express your LOVE just through words. [Talk, After Mad Mew Mew can be spared]
- True Pacifist Route
- You…! | You!!!!!!
- Thanks a lot for your help earlier, mew~
- I'm starting to feel GREAT in this new form!!! | I want everyone to see it!!
- Who knows, maybe I'll even start my OWN TV show and take the world by storm!
- Watch out, Mettaton!!! | I'll STEP on your puny metal behind!! Hahaha!!!
- … | … hey, do you think Undyne's going to come back here soon, or...?
- Isn't it NORMAL to think a lady might want to watch her own house burn down!?
- It's sort of like, er, your own personal fireworks show! VERY personal!
- If protagonist attacks Mad Mew Mew
- You thought you could stop ME from using this body!?
- I may be in pieces, but I still gotta be myself!
- Your days are numbered, mew~
- When Undyne asks if anime is real or not during the date with Alphys, the answer that the protagonist gives will alter the name of the border that is given at the end of the encounter with Mad Mew Mew.
- If the player hasn't reached that point, the border can be selected, but the name of the border will be blank.
- Due to their addition, there is now a fight in all 4 main areas of Undertale that use the Ghost Fight leitmotif (Napstablook in the Ruins, Mad Mew Mew in Snowdin, Mad Dummy in Waterfall and Muffet in Hotland/CORE.)
- "Gnarly! Tubular! Way Cool! Awesome! Groovy! Mondo! Outrageous! Funky!" is a reference to Super Mario World's level names in Special Zone.
- Her face sprite is separate from her head sprite, similar to Temmie.
- Although she is described as a life-sized doll during most of the fight, she is revealed to be an animatronic character once she is defeated violently.
- She is described as limited-edition, implying that she was made on the Surface like other Mew Mew Kissy Cutie items Alphys collects.