Yo! You're a kid too, right? I can tell 'cause you're wearing a striped shirt.
— Talking to Monster Kid in Snowdin
Monster Kid is a young monster who stops at nothing to see their idol, Undyne, in action.
Monster Kid is a yellow, reptilian monster with no arms, a tail, and spikes on the back of their head. Having no arms and frequently running everywhere, they trip and fall often. They have markings under their eyes, speculated to be black eyes from falling. One of the markings is bigger than the other. They wear a yellow shirt with brown stripes.
Monster Kid looks up to Undyne as a role model. They are an innocent monster who assumes an energetic attitude that pervades most of their actions. They seem to be a bit naive, as well as a bit clumsy, as they trip frequently. They also use words like "c'mon," "dude," and "yo" frequently, usually considered childlike ways of interaction.
Despite the protagonist notably frightening Monster Kid during the Genocide Route (once they discover the protagonist's intentions), they stand up to the protagonist with an intense expression, ready to protect the Underground, trying to act like Undyne. Choosing to spare them aborts the Genocide Route and reverts them back to a more favorable opinion of the protagonist, showing a rather forgiving nature.
Monster Kid first meets the protagonist in Snowdin. They note how both of them are wearing striped shirts, so they both must be children. However, they do not seem to notice that the protagonist is a human child.
They later sneak out of Snowdin to explore Waterfall, looking for Undyne. They follow the protagonist, not knowing that the latter is a human whom Undyne is searching for, and they feel a little jealous of the attention that the protagonist receives.
While walking across a bridge, Monster Kid finds out that the protagonist is a human and reluctantly considers the protagonist an enemy. They try to say something mean to the human, in an attempt to become enemies, but feel bad for it and start to leave.
Just as Monster Kid leaves, they trip over the edge of the bridge and hang from it (presumably with their teeth) as Undyne approaches. The protagonist must make a choice:
Help them climb back up: Monster Kid defends the protagonist from Undyne, who backs off. They decide to remain friends with the protagonist and leave for home.
Let them fall: Undyne dives down after Monster Kid to save them. She is badly hurt but refuses to rest. They then tell Undyne that she was right about the human, who just stood there waiting for them to fall. This response makes Monster Kid hate the protagonist and all humans.
Run to Undyne: Undyne saves Monster Kid and is badly hurt. They tell Undyne that she was right about the human, who was too eager to fight her to bother saving them. This response also makes Monster Kid hate the protagonist and all humans.
Run away from Undyne: Undyne helps Monster Kid up, who tells her that the protagonist was just going to get help from others.
True Pacifist Route
Monster Kid is seen cheering the protagonist on to victory in the bottom-right corner of the battle screen as they face off against Flowey. In the epilogue, they can be found in Snowdin. If saved earlier, they say they do not think Undyne is that great anymore, and that they found themselves a new idol, likely Papyrus. They are also seen during the credits, attending Toriel's school.
Despite being warned by Undyne that the protagonist has hurt a lot of people, Monster Kid approaches the protagonist in disbelief. The protagonist turns away, and as Monster Kid questions the protagonist about not answering them, the protagonist silently approaches them with a "weird expression," causing them to step back. Monster Kid begins to panic and wonders what they should do, and then stands up to the protagonist out of desperation, trying to act like Undyne, stating nervously that if the protagonist wants to continue to hurt others, the protagonist would have to go through them. The protagonist then proactively engages battle with them, with the narration simply declaring "In my way." If the protagonist dies, reloads at the unnamed save before the bridge and returns to this location without the player closing the game, they simply attempt to trick the child into turning the other way before re-engaging. This encounter cannot be escaped.
If the protagonist attacks Monster Kid, Undyne takes the fatal blow in their place. The kid escapes while harboring a shocked expression, and the protagonist fights Undyne the Undying. Monster Kid is not seen again for the rest of the game; it is assumed they evacuated with the others.
If the protagonist spares Monster Kid, they say they knew the protagonist was not that bad and leave. This action aborts the Genocide Route, and the protagonist fights Undyne later in Waterfall normally.
A slowed down version of "Anticipation" plays during the battle. If the player resets after this cutscene (most likely by dying to Undyne the Undying), the scene is abbreviated: after Monster Kid approaches the protagonist, the protagonist tricks them into turning the other way, then re-engages battle.
Yo! You're a kid too, right? I can tell 'cause you're wearing a striped shirt.[1st encounter]
Yo, everyone ran away and hid somewhere. Man, adults can be so dumb sometimes, haha... Don't they know we've got Undyne to protect us!?[Genocide exclusive]
Yo! Are you Sneaking out to see her, too? Awesome... she's the coolest right!? I wanna be just like her when I grow up... Hey, don't tell my parents I'm here. Ha ha.[2nd encounter]
Yo... did you see the way she was staring at you...? That... ... was AWESOME! I'm SOOOO jealous! What'd you do to get her attention...? Ha ha. C'mon! Let's go watch her beat up some bad guys![3rd encounter]
Yo... did you see that!? Undyne just... ... TOUCHED ME! I'm never washing my face ever again...! Man, are you unlucky. If you were standing just a LITTLE bit to the left...! Yo, don't worry! I'm sure we'll see her again![4th encounter]
Yo... did you see that?! Undyne just... ... TOUCHED ME! I'm never washing my face again...! ... yo, did you notice? She seemed really mad about something! She looked liked she was gonna blast me to pieces! But... sigh... Then she decided to put me down... Yo, there's always next time, right? Let's go! [4th encounter, if the protagonist stayed in the previous room for a certain amount of time]
Yo, you got an umbrella? Awesome![1st encounter with an umbrella in hand]
Yo... Where's the umbrella...? Ha... ha...[2rd/3rd encounter if the protagonist once had an umbrella, but put it back]
Yo...[3rd/4th encounter onward]
You can't hold an umbrella either? If you're walking anyway, I guess I'll go with you, haha...[1st encounter without an umbrella in hand]
Yo! You got us an umbrella? You're the best!(2nd encounter with an umbrella in hand)
Yo! Ready yet?[2nd/3rd onward]
Yo, I already looked, Undyne's not over here. So I guess I'll see ya later, haha.[if the protagonist goes left to the umbrella room]
See ya later![if the protagonist goes left again onward]
Man, Undyne is sooooooo cool. She beats up bad guys and NEVER loses. If I was a human, I would wet the bed every night... ... knowing she was gonna beat me up! Ha ha.[1st monologue]
So, one time. We had a school project where we had to take care of a flower. The king - we had to call him "Mr. Dreemurr" - volunteered to donate his own flowers. He ended up coming to school and teaching the class about responsibility and stuff. That got me thinking... YO! How COOL would it be if UNDYNE came to school!? She could beat up ALL the teachers!! Ummm, maybe she wouldn't beat up the teachers... She's too cool to ever hurt an innocent person![2nd monologue]
Yo, this ledge is way too steep...[Monster Kid at the ledge]
Hmm...[Talk to Monster Kid before heading left]
Hmm... Yo, you wanna see Undyne, right...?[if talked to/if the protagonist heads left]
Put up your umbrella and climb on my shoulders.[Leaving with umbrella in hand]
Yo, you aren't going to be able to climb with an umbrella.[Talked to again with an umbrella in hand]
Climb on my shoulders.[if the protagonist goes left]
Yo, you go on ahead. Don't worry about me. I always find a way to get through![Monster Kid after the protagonist climbs the ledge]
Owww... Yo, try being a little less rough, haha. Anyway, I'll look for another way through. Later![Monster Kid after the protagonist climbs the ledge during the Genocide Route]
Undyne!!! I'll help you fight!!! YO!!! You did it!!! Undyne is RIGHT in front of you!!! You've got front row seats to her fight!!! ... Wait. Who's she fighting??? H-hey! You aren't gonna tell my parents about this, are you?[6th encounter]
Yo, I know I'm not supposed to be here, but... I wanna ask you something. ... Man, I've never had to ask anyone this before... Umm... Yo... You're human, right? Haha. Man! I knew it! ... well, I know it now, I mean... Undyne told me, um, "stay away from that human." So, like, ummm... I guess that makes us enemies or something. But I kinda stink at that, haha. Yo, say something mean so I can hate you? Please? [7th encounter]
Huh...? Yo, that's your idea of something mean? My sister says that to me ALL THE TIME! Guess I have to do it, haha.[Yes]
Yo, what? so I have to do it? Here goes nothing...[No]
Yo, I... I hate your guts. ... Man, I... I'm such a turd. I'm... I'm gonna go home now. Yo, w-w-wait! Help! I tripped!
Undyne... You... You saved me! Huh, They ran away? Yo, you're wrong... They went to get help! They'll be back at any second!! O-okay, I'll go home...[Protagonist heads to right; Undyne jumps off after Monster Kid]
Wh-what are you standing around for? get over here, dude![Protagonist does nothing]
I... I'm slipping![Monster Kid falls, and Undyne jumps after them]
Undyne... you saved me...? Yo, I thought I was a goner, haha... ... wait, are you okay? It looks like you fell pretty hard... Th-this is my fault. I should have stayed away from them, like you said.[Protagonist continues to do nothing]
They just went straight to fight you instead of helping me... I was so scared, and you... What? You're gonna go fight them now? But you look hurt... You should rest, haha... ... W-warriors don't rest huh? Undyne... you're really cool.[Protagonist rushes toward Undyne; Undyne jumps off after Monster Kid]
They just stood there... Watching... Waiting for me to fall.[Protagonist did nothing]
Y... y... yo... dude... If... if y-you wanna hurt my friend... You're gonna have to get through me, first. She's gone... Yo, you really saved my skin. Guess being enemies was just a nice thought, haha. We'll just have to be friends instead. ... man, I should REALLY go home... I bet my parents are worried sick about me! Later, dude![Protagonist saves Monster Kid]
True Pacifist Ending Epilogue
Yo... There you are!
I've been kinda worried about you since you ran away. ... But now i'm just kinda mad, ha ha. If Undyne hadn't saved me, that woulda been bad...[If the protagonist ran away from Undyne during bridge sequence]
...[Talked to again]
Yo... Uh... Don't talk to me.[If the protagonist lets Monster Kid fall from the bridge]
...[Talked to again]
I've been kinda, doing some thinking... Maybe Undyne... Isn't actually as cool as we thought. She's just kinda... mean. But YO!!! I just found out about someone WAYYY cooler!!! Nyeh heh heh!!![If protagonist saved Monster Kid during bridge sequence]
Nyeh heh heh, dude.[Talked to again]
Yo. Undyne told me to stay away from you. She said you... You hurt a lot of people. ... But, yo, that's not true, right!? ... (Protagonist turns away) ... yo... Why won't you answer me? A... a... and what's with that weird expression...? (Protagonist steps toward Monster Kid, who steps back) Oh... Oh man... (Man, my h-heart's pounding right out of my chest...) (... what would Undyne do?) (...) Yo... Y-you'd b-better st-stop r-right where you are... Cause if you w-wanna hurt anyone else... you're... You're gonna have to get through me, first. A... an... and... (Engage battle)
S-see? I knew y-you weren't so bad! I've gotta go tell Undyne she was wrong about you![Protagonist spares Monster Kid]
Undyne... You're... You're hurt...[Protagonist attacks Monster Kid; Undyne arrives and take the blow]
[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![Monster Kid leaves]
Yo, dude... I... I heard some things, and... What? Look over here? What for (Engage battle) [Post-introduction, subsequent resets]
Looks like free EXP.[Check]
Monster Kid quivers anxiously.[Neutral]
In my way.[Encounter]
Monster Kid's tarot card as Major Arcana XIX, "The Sun".
The Pokémon Scraggy might be the inspiration for Monster Kid's design; both are small, yellow, bipedal lizard-like creatures with protruding teeth. While Scraggy may have arms, it has to use them perpetually to hold up its pelt, which is a lot like not having arms at all, just like Monster Kid.
Monster Kid's assumption that the protagonist is a kid because of their striped shirt may be a reference to the MOTHER series, where in the main protagonists of each game are kids who wear striped shirts.
Monster Kid can be seen behind Undyne the Undying's head in the "The Undying" tarot card.
In the Japanese translation, Monster Kid uses "ore" for themselves, which is commonly used for males.
Monster Kid is referred to as male in the Art Book.