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In Demon Slayer, practically all demons possess a murderous instinct to kill and devour humans, and have lost most of their memories from their human life. They only care about eating humans, even eating their own family.
However, there are a few demons who have completely retained their human emotions and memory. Some of them can control their desire to feast on humans. Let’s see who they are.
1) Muzan Kibutsuji
Originally a human, Muzan became the first ever demon over one thousand years ago. Throughout his young life, he struggled with illnesses and that led to his thanatophobia. As a young man, Muzan was diagnosed with a disease that would kill him before he reached the age of 20. The doctor treating Muzan gave him a special prototype medicine, but it appeared to have failed. Enraged by his worsening condition, Muzan killed him.
Only after killing the doctor did he realize that the medicine was actually working and had helped him acquire a strong body. He realized he needed to consume the flesh of humans to survive but was not bothered by this fact. His new, more serious problem on the other, was his inability to walk in the sunlight, as doing so burned him severely.
It was during this moment that he truly regretted killing the doctor before he could have finished his treatment with his “Blue Spider Lily” medicine. And only the doctor know where to find the “Blue Spider Lily”.
After becoming a demon, Muzan has always retained his human memories and emotions. But as a demon, he also became lack of empathetic.
He searched the entire country for the flower, but couldn’t find it. Due to this, Muzan started to create more Demons and planned to spread them in the world, hoping that one of them would eventually find a way to overcome the sun or at least help him on his quest to find the Blue Spider Lily.
Kokushipo was born almost 5 centuries ago and his birth name was Michikatsu Tsugikuni. Michikatsu had a younger twin brother and the twins name was Yoriichi Tsugikuni, the same Yoriichi who would go on to become the strongest demon slayer and original user of sun-breathing.
They came from a prominent samurai family during the Sengoku period. Having twins was actually considered a bad thing since they are born at practically the same time which could result in fights over who will inherit the father’s land and titles.
Michikatsu is the older brother, he was groomed to be the head of the family. Yoriichi is the younger one, the family would raise him until the age of 10 and they would send him away to a temple where he would become a monk. Due to the fact that monks usually have to renounce all of their worldly possessions, Yoriichi would no longer have any claim to the family’s land and titles.
They brothers both want to be strong Samurai. Yoriichi is born with demon slayer mark on face. He could see into the transparent world. So he could anticipate his opponent’s movements based on the position of their bones, the contractions of their muscle, the flow of their blood and so on. Michikatsu also worked hard on training but his progress was nothing compared to Yoriichi who was just an unrivaled genius.
Later Yoriichi left home and Michikatsu succeeded his father. Next, they met each other in ten years. Michikatsu was almost killed by a demon in the forest and Yoriichi appeared and killed the demon instantly. Michikatsu knew Yoriichi had mastered the sword and was unbelievably strong.
Michikatsu decided that he would do anything to become as strong as skilled as his brother. Michikatsu went so far as to abandon his family and dedicate his whole life to following in Yoriichi’s footsteps and becoming a demon slayer.
Michikatsu trained under Yoriichi. And he learn breathing technique from Yoriichi and change slightly, made his own moon breathing. Eventually, Michikatsu manifested a demon slayer mark identical to the one that his brother was born with.
All the demon slayers who had managed to manifest the demon’s hair mark began to die off after reaching the age of 25. The demon slayer mark allowed the slayer to unlock power, but in exchange for that power, it drastically reduced the demon slayer’s lifespan.
Michikatsu realized that he was going to die without ever even coming close to achieving his dream of becoming the strongest Samirai and surpassing his brother. He is so lost, confused and jealous.
And in that moment of weakness and desperation, he was approached by Muzan, Muzan told Michikatsu that if he became a demon, he could live forever and he wouldn’t have to worry about dying at the age of 25. He would also have a much stronger body and he would have all of eternity to become stronger than Yoriichi. And so, even though he dedicated his whole life to ridding the world of demons, Michikatsu’s Jealousy got the best of him. He agreed to consume Muzan’s blood and become a demon, in exchange for eternal life and super human power, that is when Michikatsu Tsugikuni became Kokushipo.
He remembered everything when he was human. When Kokushipo fought with hashiras in Infinite Castle Arc, he took it upon himself to say who he was. His birth name was Michikatsu Tsugikuni, brother of the Yoriichi, as well as the ancestors of Muichiro Tokito. He said he missed the days of hunting demons with his brother. And he told stone hashira Gyomei Himejima, the demon slayers who had managed to manifest the demon’s hair mark would die off after reaching the age of 25.
Doma was born with insanely rare rainbow colored eyes and pristine white hair. His parents convinced Doma could hear the voices of gods, and they created the paradise faith religion around him. Doma never heard the voices of gods. He found it ridiculous that fully grown adults would believe this stuff and worship him even though he was just a clueless kid.
Normal human emotions meant nothing to doma even before he became a demon.
While doma was still fairly young, his mother found out that his father was cheating on her with female followers of the cult, she flew into a rage and stabbed him to death before taking her own life. But Doma just thought that his mother shouldn’t have made such a mess in the room. And that someone now has to clean up all that blood.
Doma always had sociopathic tendencies and this was only exacerbated once he met Muzan and became a demon at the age of 20.
After becoming a demon, Doma believes that he is actually saving people by devouring them. Because everyone fears death, they could just live forever through him after he devours them, and his victims are no longer afraid of anything or suffering in any way after they are eaten.
In the Infinity Castle arc, Doma tells the story of Kotoha, Inosuke’s mother, and how she and her baby son joined Doma’s cult only to end up running away after she realized the Demon is eating his followers.
Doma can stick his finger through his skull and into his brain to try and jog his memories.
Kaigaku is the disciple of former thunder hashira Jigoro Kuwajima and trained to use thunderbreathing alongside with Zenitsu. He also is the second demon slayer to become a demon after Kokushibo.
Kaigaku was an orphan who led a very tough childhood. He had to become a thief and steal scraps of food just to survive. At some point he was taken in at a temple where he lived with eight other orphans.
Later he broke the rules of the temple and didn’t return before dark. Kaigaku captured by a demon in the forest, in order to save himself, Kaigaku could pleaded with the demon to spare him and in return for sparing his own life, he promised to lead the demon to the eight other children at the temple.
Kaigaku extinguished the wisteria flower incense and let demon enter the temple. Because of Kaigaku’s betrayal , seven children were taken out by demon. But Kaigaku himself survived.
His rough life had led him to conclude that in order to survive, he must be willing to do literally anything and what happened to other people did not matter as long as he came out on top in the end. This extreme and cruel world view would continue to play a major role in Kaigaku’s life.
Years later, he was taken in by former Thunder Hashira Jigoro Kuwajima and trained to use thunder breathing and finally become a demon slayer. Kaigaku was able to master all of the thunder breathing forms except for the first form, which is the only form that Zenitsu was able to master. He hated Zenitsu because he saw him as weak and cowardly. He often mocked and bullied Zenitsu during their training.
One day Kaigaku came face to face with Upper Moon 1 Kokushibo as a demon slayer, Kaigaku had the duty to fight Kokushibo but after sensing Kokushibo’s overwhelming strength, Kaigaku simply dropped down to his knees and began begging and pleading with kokushibo to turn him into a demon. Kaigaku once again proved that he had no other values or principles beyond survival at all cost as soon as he felt that his life was in danger. He immediately betrayed his duty as a demon slayer and groveled before a demon just as he had done as a kid back when he betrayed his fellow orphans in order to save his own life.
Kokushipo agreed to turn Kaigaku into a demon. And once he became a demon, Kaigaku readily devoured a large number of people in order to get stronger, he was eventually promoted to the rank of upper moon 6.
The fact that one of his disciples went on to become a demon was so humiliating for Jigoro that he committed seppuku with no second in order to atone for his disciples crimes.
In the Infinite Castle, Zenitsu told Kaigaku that Jigoro seppuku because of him, Kaigaku was also indifferent, and instead blamed Jigoro for not teaching all the technique of thunder breathing to him. This also proved that even if Kaigaku became a demon, he kept it Human memory and selfishness.
5) Lady Tamayo
When Tamayo was a human except that she was a married woman with children. She fell ill with a life-threatening disease and began seeking a cure, which is how she first came across Muzan Kibutsuji. Muzan didn’t tell the truth to Tamayo and just offered to treat Tamayo’s illness. She naively accepted. Later Muzan turned her into a Demon and left her to run rampant around the town. The lose-minded demon Tamayo killed her husband, children, and a number of other humans.
Tamayo hated Muzan. She also tried to kill Muzan. But she obviously lose the battle, there is massive difference in power between Tamayo and Muzan. Finally her spirit completely broken by Muzan and she became his servant.
But later Yoriichi occured, Tamayo saw the Yoriichi completely overpowering Muzan and nearly dealing a death blow on him. Her spirit to kill and defeat him was reignited, it gave her hope that Muzan could be killed.
Yoriichi spared Tamayo and connected with her over their shared hatred for Muzan. He let her escape. She recalls that Yoriichi said that he believed in her wish to defeat Muzan.
Tamayo somehow managed to survive through consuming animals and human corpses. She fought her internal instincts and refused to attack humans, despite all the pain she felt. A period of time elapses, so Tamayo regained her sanity and proceeded to set up a life in which she could study the biology of demons and seek a cure. After 200 years of study, Tamayo becomes capable enough with medicine to modify her body so that she can function without eating humans.
Lady Tamayo is the first demon escaped Muzan’s control. She retained his original personality and belief, revenge on Muzan for his husband, and children.
6) Nezuko Kamado
Nezuko still remembered her family. She still retained her human emotions, as she had been seen to cry and be happy. She always wanted to protect her brother Tanjiro. This proved that she still was a conscious demon.
But she doesn’t retain all her human memory. She doesn’t remember how Muzan kill her family. In the final Season, when she turns into a human again, she remembers Muzan attacking her house and killing off her family to try and create a demon that can withstand sunlight. After this, she regains all of her memories from her journey as a demon.
Do Other Upper Moon Demons Retain Their Human Memory?
I think it honestly has to do with how traumatic their life was and the reason they chose to turn.
Every demons whose memory got wiped had some traumatic experience go on or had a regret that haunted them.
While those demons that retain their memories were plain assholes. They do still retain their original personality, most times it becomes more ruthless criminal of what it used to be.
Upper Moon 3 Akaza
In Demon Slayer: Infinity Castle Arc, Akaza remembered his past when he was regenerating his head after Tanjiro cuted it off, During this time, he remembers his sorrowful past.
At the last time of his life, he was driven by grief and rage for having lost the two people he cared about, he goes to rival dojo and murders all 67 people there. Muzan goes there as well, because he heard rumors of a demon. He explains to Akaza his plan to make the 12 demon moons, and then stabs him in the head. He tells him that if he can survive this amount of blood, then come and meet him.
Because Akaza lost his will to live when his fiancee died. His last words to muzan were “I don’t care..” as a human. Muzan couldn’t bear the fact that someone who was so gifted and powerful has lost his will to get stronger so he altered his memories.
When Akaza remembered his late fiancée, he became remorseful over the monster he’d become and obliterated his own freshly regenerated head, allowing himself to die.
Upper Moon 4 Hantengu
In the second Demon Slayer Fanbook, Hantengu’s backstory is revealed. Even as a child, Hantengu lied to make those around him look bad and gain sympathy. He was actually a ruthless criminal who did many terrible things throughout his long life. He stole from people and he also murdered people in towns and cities across Japan.
Because he was a notorious and violent criminal, Hantengu often changed his name, age, and background so much that he forgot about what his real name, age, or background really is.
Hantengu has married and had children several times. This didn’t last though as whenever he would be accused of dishonesty, he would feel as if he was mistreated and kill his wives and children in anger.
Hantengu was apparently around 87 years old when he became a demon. And this happened after he was finally captured by a magistrate and sentenced to death for his crimes.
As Hantengu sat in jail awaiting execution, he was approached by Muzan who offered to turn him into a demon. Hantengu agreed. After becoming a demon, he escaped prison and went to get revenge on the magistrate who had condemned him to death.
Upper Moon 5 Gyokko
Gyokko not much is revealed about his backstory in the actual manga itself. He was the only Upper-Rank demon who 1v1’d a Hashira and lost. Unlike other demons, Gyokko didn’t regain his human memories after being decapitated.
In the second Demon Slayer Fanbook, we get some useful information. When Gyokko still was a human, his name is Managi.
He was an outcast who lived on the outskirts of a fishing village. He was considered to be a weirdo by everyone since he apparently collected fish carcasses and did other weird and creepy things. Managi lost his parents at a young age, they were both fishermen. On one of their fishing trips, they had an accident and drowned. Once the bodies of his mother and father were uncovered, they were badly decomposed, but bizarrely, Managi thought that the bodies were beautiful and he became fascinated by them.
Some times afterward, a village children came to tease him for being so weird. Managi killed the child and stuffed the body in a jar. When the child’s parents found out, they stabbed Managi with a two-pronged fishing fork and left him to die. But Managi was stubborn and he clung to life nearly half a day after being wounded, and Muzan was passing by, so he turned Gyokko into a demon.
Upper Moon 6 Daki and Gyutaro
Daki and Gyutaro don’t retain all their human memory. Gyutaro does still retain his original personality and belief to protect his sister.
In Demon Slayer Entertainment District Arc, Gyutaro sneers at Tanjiro who can’t protect Nezuko, thinking that a brother who can’t protect his sister is a loser. But Gyutaro does not remember that when he was human, he was not able to protect his sister either. He only retain the personality and despises people who can’t protect their sisters from the deep heart.
For some demons living hundreds years old, it is time itself that determines the loss of their memories as humans. Technically it’s not loss just buried so deep that only in death can they remember.
All demons do regain their memories moments before they die, which possibly means that demons also have their lives flash before their eyes before they turn into complete ash.