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Underdecay is a world where the constant resets actually start to affect not only the characters, but HOME itself. No matter the path, kindness or violence, every reset leaves a permanent mark against the new timeline, similar to a scar that never fades.

Each reset leaves a small impression on the monsters that interact with the fallen human, the result of which becomes more and more prevalent with each new reset. The monsters are slowly becoming more aware about living the same life over and over as moments of deja vu and unexplained intuition seep into their lives. The past Royal Scientist took it upon himself to try to mitigate the damage caused by perpetual resets and for the most part actually succeeded, but in his absence, the world slowly falls into decay.



  • He enjoys exploring his new playground and discovering the new anomalies around the underground.
  • He’s not a threat to monsters anymore.
  • He actually attempt to stop the fallen human from making True Resets by using the 6 humans souls.
  • He gets along with Sans.


  • She’s the main character that died the most.
  • She turns to dust a few hours after the resets.
  • She doesn’t react to outside stimulus anymore.
  • Decay!Sans is aware of her state.


  • He’s keeping himself isolated from most monsters.
  • He mostly stays at his home listening to music.
  • He keeps crying no matter what.


  • New stimulus confuse him and makes him anxious.
  • The growing anxiety and worry with each reset starts to affect his decisions and his reaction as well.
  • He’s scared of physical contact (due to having been killed a few times) and doesn’t talk very much except if he needs to. (Like many, he bottles himself up.)
  • When relaxed, he’s pretty close to the Sans we all know (especially when it comes to bad puns!).
  • His eye changes color from yellow to cyan depending on his mood.

Gaster Blasters

  • They act pretty much like the classic ones aside from few details.
  • They’re ‘sentient’ and act through the subconscious of Sans.
  • They can’t form properly and, therefore, tend to miss parts here and there.
  • Only their right eyelight works, and just like their owner, they glitch.


  • Papyrus seems like he still has his positive attitude.
  • He partially remembers resets and sees them as déjà-vu and livid dreams.
  • He feels confused when things are not where they're supposed to be, causing him lots of stress every time something is out of place.

Canine Unit


  • Like all of the Canine Squad, Doggo works for Undyne.
  • He’s loyal to the Royal Guard and tracks down the fallen human.
  • His sight is pretty bad, he mostly uses his nose.
  • Pet won’t affect him.

Lesser Dog

  • He’s loyal to Undyne and works alongside her to hunt the fallen human.
  • His main role is to protect monsters from any possible aggression.
  • Any attempt to pet him will fail, neither will he try to play.
  • He uses his long neck to attack at long distance.

Greater Dog

  • He’s loyal to Undyne and works alongside her to hunt the fallen human.
  • The smell of the fallen human makes him agressive.
  • Any attempt to pet him will fail, neither will he try to play.
  • He protects Undyne and his team against monsters.


  • She’s blind and only uses her nose to guide herself and her attacks.
  • She’s really alert and works with Undyne to hunt down the fallen human.
  • She always stays close to Dogami and became dependant of him.
  • She obey sblindly to orders given to her and won’t hesitate to kill monsters.


  • His left eye got too damaged to see properly and has no depth perception.
  • He’s overprotective towards Dogaressa and tends to be agressive.
  • Like Dogaressa, he works with Undyne to hunt down the fallen human.
  • He won’t hesitate to kill monsters going against Undyne’s order.

Annoying Dog

  • Annoying Dog still bothers Papyrus when he is fighting.
  • He’s the ‘unwanted' pet companion of the skelebros.
  • Sans used to pet him and encourage him to be mischievous.
  • He doesn’t seem much affected by the reset as this dog had never been killed.


  • Still pretty silent and reserved like his classic version.
  • Is slightly affected by the absence of MANY of his common customers.
  • Is pretty hostile towards the human.

Snowdin NPC's

Scarf Mouse

  • He remained pessimistic.
  • He became extremely anxious and paranoid.
  • He stays away from the fallen human but can be aggressive if needed.

Naracat Jester

  • His positive attitude remains, however, he is rather worried about the decaying state of Home.
  • He stays away from the fallen human and fights only to defend his life.
  • Like most monsters, he became rather insecure about the future of the underground.


  • She’s more violent and aggressive than she used to be.
  • Her magic is pretty much unstable and tends to turns partially against her.
  • You can’t befriend her anymore.
  • She’s determined to gather the last human soul at all cost.
  • She encourages other monsters to do the same as well, but works on her own.

Monster Kid

  • Monster kid is more wary towards the fallen human.
  • He has bad impression of deja vu.
  • His admiration for Undyne developed into fear.

Echo Flower

  • They barely glow anymore.
  • Most monsters avoid their presence.


  • She’s terrified of other monsters.
  • She isolates herself.
  • She became mute (psychologically unable to speak.).

Mad Dummy

  • He’s really agressive.
  • He tends to stab you when you’re not looking.
  • He makes sure he has more then one knife.


  • You can find her in Temmie village.
  • Temmie Flakes is one of the rare kinds of food that actually heal.
  • She kept her cheerful personality and doesn’t seems conscious of resets.


  • She’s cold and calculative.
  • She prefers to spend her time in her personal laboratory to focus on what matters the most: her DT experiments.
  • For the greater good of all monster kind.


  • Like UT!Mettaton, the metal part of his body is extremely durable, making it hard to take him down.
  • The screen and the soul container is damaged however, making it hard for him to coordinate his body properly.
  • Mettaton’s vision is slightly damaged, but he has gotten used to it.
  • His main goal is to stop the fallen human and gather their soul at all cost.


  • Her food is some of the only food in the whole Underground that can actually heal.
  • She’s not human friendly anymore even if they bought her food.
  • Her main goal is to gather the last human soul at all cost.
  • During her fight, she doesn’t give the fallen human any of her merchandise.


  • Endogeny, Snowdrake’s mom, Reaper bird, Lemon bread, Memory Head, despite falling apart, remain intact whenever a reset occurs.
  • They are nothing more than test subjects for Alphys.
  • Their former identities vanished long ago.
  • They’re close to feral, only a glimpse of intellect seems to be there.
  • They’ll try to eat the fallen human if they ever go to the secret lab.


  • He lost his will to fight the fallen human.
  • He tries to reason with them rather than to attempt to take their soul.
  • He’s eaten by guilt.
  • He doesn’t take care of his plants anymore.


  • He doesn’t want Chara to keep playing anymore.
  • He tries to gather the fallen human souls to free everyone.
  • Asriel is an important ally to the inhabitants of Home.

God of Hyperdeath

  • His form constantly break apart and put itself back together.
  • He speaks for the name of the underground.
  • The human souls are his allies.


  • He’s the past royal scientist that has been replaced by Alphys when this one ‘fell into his creation’.
  • No ones remembers him.