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Jerome is a mussel in the Underground. He is the main companion of Onionsan.


Jerome is a mussel who lives in Waterfall with OnionSan. Like other monsters, he is able to talk, but he can live both underwater and on land. However, he tends to avoid Hotland since he is afraid of hot objects.

Jerome looks like an ordinary mussel that can talk. He has no arms or legs, relying rather on his jumping to move from one place to another. He does have eyes that appear to be detached from his body.

Jerome is incredibly fast, whether in water or on land. Most monsters report that they have a hard time trying to catch him whenever he is on the run. This is an unusual trait, seeing that he has no legs.

Jerome tends to be a vigilante, believing that the law is too slow. However, he shares a warm relationship with Undyne, even though he does not want to be a part of the Royal Guard. He is often seen with a ski mask, even though the mask is unnecessary. He will fight, however, for what he believes is best for the Underground. He tends to be arrogant in most cases.



  • Onionsan
  • Undyne


  • Gerson
  • Alphys
  • Mad Dummy
  • Woshua
  • Napstablook
  • Mettaton
  • Papyrus
  • Sans


  • Flowey
  • Aaron



  • HP: 100
  • AT: 10
  • DF: 12
  • EXP On Kill: 1000
  • Gold on Win: 100


OPTIONAL* If your character changes stats depending on the run.

  • HP: 10000
  • AT: 65
  • DF: 86
  • EXP On Kill: 10000
  • Gold on Win: 1000


List ACTs here.

  • Catch
  • Flex
  • Feed

NOTE: These actions are available ONLY in neutral/pacifist routes. Genocide routes have only CHECK.



  • "Wait, what are you doing here? You want to beat me? Ah, sucker! You need to catch me first!!!" [first encounter]
  • ATK 10, DEF 12. The fastest mussel in the world. [check]
  • "I am the fastest monster in the Underground!" [neutral]
  • "I can beat you in a race easily!" [neutral]
  • "Even Sans can't teleport as fast as I can glide!" [neutral]
  • "You're too slow!" [catch]
  • "Woosh! Is that your fastest?" [catch]
  • "Excuse me, are you Aaron?" [flex]
  • "Dude, I have no 'mussel'... heheheheh." [flex]
  • "Um... food?" [feed 1st time]
  • "Wow... I am kinda hungry..." [feed 2nd time]
  • "Why not? I want food. You're cool! Thanks!" [feed 3rd time]
  • "So... you came. After slaughtering everyone, are you going to slaughter me? Onionsan? Undyne? No... I will defeat you!" [encounter in genocide]
  • "I have the power of speed! No strength can outlast that!" [Attack]
  • "So who's next? My best friend, Onionsan?" [Attack #2]
  • "Yeah, no. This law may be too slow, but I'll never be!" [Attack #3]
  • "Even if you wanted to kill my best friend, I've hidden him." [Attack #4]
  • "Even if this place is a small world, there are multiple hiding places." [Attack #5]
  • "Now as to why I never attacked... a vigilante will never run." [Attack #6]
  • "I'll kill you before you can get to Undyne!" [Attack #7]
  • "Time to stop your rampage once and for all!" [Attack #8]
  • "Trust me, that weapon you carry, you'll never get me." [Attack #9]
  • "You murderer... you killed Papyrus, Snowdrake, Ice Cap, Woshua... everyone..." [Attack #10]
  • "You should see what Sans has in store for you. It won't be good for you." [Attack #11]
  • "Ow... well... ... I... didn't expect that... I tried, anyway. At least my best friend won't die to you. No. He's safe! At least I can die happily for that!..." [Attack #12]

Flavor Text

Insert flavor text here, the flavor text in bold and the trigger in [brackets].

  • Jerome appears!... flashes... [encounter]
  • Jerome is continuously splashing from one side to another. [neutral]
  • Jerome is checking how fast you are, and decides that you're too slow for him. [neutral]
  • You flex at Jerome. Jerome thinks you're Aaron. [flex]
  • You flex at Jerome. Jerome laughs at his own joke. [flex]
  • You try to catch Jerome. He continuously slips between your hands. [catch]
  • You show Jerome some food. He seems tempted. [first time feed]
  • You show Jerome some food again. He seems more tempted. [second time feed]
  • You show Jerome some food again. He snags it out of your hand. He seems content. [third time feed]


  • Jerome apparently hates Aaron since Aaron keeps bragging about his muscles
    • This is hypocritical, since Jerome himself brags about his speed.
  • Jerome is significantly buffed in a genocide route as he turns from boast mode to vigilante mode.
  • Jerome takes less attacks to kill than Sans; however, the fact that he has a high ATK makes it hard to achieve this.
  • Jerome claims to be faster than Sans' teleportation
    • If this were true, he would've easily went from Snowdin to the Judgement Hall. Seeing as even the speed of sound cannot travel this quickly, this should be impossible
  • Jerome often hangs out with Onionsan.
  • Jerome idolizes Sanic, as can be seen in one of his attacks.
  • Jerome is inspired from Mussels Marinara, a mussel in Mutts.

In other AUs


Jerome lives in an aquarium in the Party Ground. He left Onionsan, but occasionally visits him to hang out. Jerome is one of the entertainments in the party. He lives with other fishes and underwater dwellers.


Jerome died in Waterfall after witnessing the death of his best friend, Onionsan, in the lake. He thus took revenge as a vigilante, attempting to slowly kill Justice. After Justice came back to rid the underground of monsters, he attacked. At first, it seemed to be working: Justice took blow after blow, with his bullets just missing. However, Patience and Integrity came, and with their combined power, they were eventually able to catch Jerome and crush him with a rock. He laid there, bleeding out, but smiling as he knew he had tried his best, and Justice and the souls moved on to Shyren.


First arc: Jerome was one of the few noticeably different characters. Whereas he used to be a boaster or vigilante, he was here a sworn enemy of all humans, as he mistakenly blamed Justice for causing the death of Asriel, and ended up as a supporting antagonist. He still like Onionsan, who was oblivious to Jerome's change in character.

Second arc: Jerome's character deteriorated to the point where he is now a crazed serial killer. He attacks anyone who happens to be outside when they're not supposed to be, and is responsible for the unofficial martial laws. Everyone really hated him, but he was feared amongst everyone. No one could catch him, and he would continue to hate everyone.

Third arc: Jerome is delusional and insane, but he hates Flowey, believing him to be an even worse ruler than Asgore, and thus indirectly joins the Against against his will. He hates the Rebellion, as he believed that there was no point in trying to kill Flowey, and that he was never accepted into the group. He forms his own group, the Uprise, in which he is the only member. Eventually, he is killed trying to kill his best friend Onionsan, and is crushed between two rocks.


Thanks to PixelzOnYoutube for putting up the ResurrecTale storyline for Jerome!

Thanks to II Finale II for putting up the Petaltale storyline for Jerome!


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