As a Human
Because of his beautiful, rainbow-colored eyes, Doma is seen as a supernatural being since birth and is groomed into the religious leader of the Paradise Faith cult, a position he maintains to the present day.
When he was still a child, his mother had a mental breakdown and stabbed his father to death for having numerous affairs with many other female cult disciples. She committed suicide immediately afterwards by poisoning herself to death. However, despite these series of gruelling events happening right in front of him, Doma only felt annoyed by the bloody mess created in the aftermath and was disgusted by the pungent smell of blood. He simply wanted to air out the room before the stench permanently stuck to the furniture.
As a Demon
At some point, Doma became Upper Rank 6 of the Twelve Kizuki. During that time, while he was roaming around the Red Light District and feasting off the flesh of the geishas and other young maidens, he stumbled across the siblings Gyutaro and Daki, who were both at death's door—Gyutaro had been stabbed in the back and is slowly bleeding to death while his sister Daki had been burned to the point of being unrecognizable. Being a "nice guy," Doma offered to turn the pair into demons and save them, proceeding to give them his blood to initiate their transformation. He challenges them to become powerful enough to be recognized by "That Man" and become an Upper Rank in the Twelve Kizuki. At some point, Doma became Upper Rank 2, surpassing Upper Rank 3, Akaza, despite becoming a Demon more recently than him.
Fifteen years prior to the present, Doma had a brief history with Inosuke Hashibira. His mother, Kotoha Hashibira, escaped from her abusive household with his father and found refuge in the Paradise Faith cult. Kotoha's husband and mother-in-law entered the cult too, but Doma told them they were too noisy, murdered them, and abandoned their corpses in the mountains.
Although Doma considered Kotoha stupid, her beauty and singing talent convinced Doma not to consume her and to simply let her live her life. However, she discovered he is devouring other followers and fled from the temple with Inosuke in her arms. Before she could be killed, she dropped Inosuke off a cliff into a river in hopes he would survive. Doma shortly caught up with them and proceeded to swiftly kill her and mock Inosuke for his fate, before eating Kotoha to the very bones.
A few years before the present, he encountered and fought Kanae Kocho. He mortally wounded her, but is forced to retreat from the rising sun before he could devour her.
At the gathering of the Upper Ranks, Doma questioned Akaza's lack of concern for him, placing a hand on his colleague's shoulder. He expressed his worry for his comrades' safety, before engaging Gyokko in small talk about his pot and the one he had been given. An irritated Akaza told Doma to move his hand, punching him when he didn't immediately comply. Regenerating the damage almost instantly, Doma complimented his fellow Upper Rank, wondering if he had gotten stronger since their last meeting. Soon after, Muzan arrived and Upper Rank 2 listened as his leader reported Gyutaro's death. Doma apologized, eagerly wondering what kind of punishment Muzan would inflict on him, but is instead told that nothing is expected of him.The eccentric Demon questioned this, but Muzan pointed out the continued existence of the Ubuyashiki family and the failure to find the Blue Spider Lily caused Doma to concede that he isn't the best at detective work. He later approached Gyokko, eager to accompany him on his new mission, but had the top half of his head destroyed by Akaza for not following their leader's orders.
Doma swiftly regenerates from this and attempts to placate Kokushibo, who had cut off Akaza's hand for disrupting the hierarchy. Hearing it hadn't been for his benefit, Doma noted that Kokushibo's suggestion that Akaza should apply for a bloody battle would be a waste of time given their difference in strength and that he hadn't dodged Akaza's blow on purpose. He then cheerfully bid Kokushibo goodbye, again attempting to engage Akaza in conversation, and once more tried to join Gyokko and Hantengu on their mission. Failing on both accounts, he attempts to flirt with the Biwa Demon, only to be rejected and transported back to his home, where he is told that his followers had arrived. Putting on a ceremonial hat, he instructs the messenger to bring them in.
As he is consuming a group of his female disciples inside Nakime's Infinity Castle, Doma notices Shinobu at the entrance to his room. He reacts with delight at having another meal walk right into his room, and thanks Nakime. One of his disciples, still alive, begs Shinobu to save her, and Doma, irritated by this, attempts to finish her off, only for Shinobu to jump in and save her. She asks the young woman if she is alright, but before she can answer Shinobu's question, the young woman explodes and dies right in front of her eyes.
Angered by this, and Doma's insistence that he saved the girl by killing her, Shinobu notices a pair of sharp fans that Doma uses as his main weapon, and angrily inquires if he remembers the haori that originally belonged to her elder sister. Doma finally remembers her by the butterfly wing design haori that Kanae once wore, and playfully expresses his disappointment at not being able to eat Kanae due to the sun rising, which further angers Shinobu. The Insect Hashira swiftly launches an attack at him using her Insect Breathing, Dance of the Bee-Sting: Mere Fluttering to inflict a powerful stab directly in Doma's right eyeball, which impresses him since he couldn't block it.
Although he is initially dismissive of her, since he correctly guessed that she lacks the physical strength to cut off a Demon's head, he becomes excited upon learning that Shinobu's true strength lies in her use of various poisons to kill her opponents. Shinobu initially hoped that the first dosage of poison she injected into him would be effective enough to kill him, but then learns, much to her disappointment, that Doma had already received information regarding her poison from her battle against the Spider Family of Lower Rank 5. Doma then manages to break down and render her poison non-lethal, healing himself from her attack. She is further backed into a corner due to Doma now rapidly developing antibodies and adjusting his body against her poison so that, after a mere five slashes and injections of poison, he is now resistant to it entirely. Also, due his Blood Demon Art of frozen mist, the air around the room they're fighting in has been growing gradually colder, causing it to become increasingly difficult for Shinobu to breathe.
Now hedging her bets on injecting a large dose to kill him by using a six stab attack, Shinobu strikes him using Insect Breathing, Dance of the Dragonfly: Compound Eye Hexagon and successfully hits him. However, as she attacks him, he simultaneously attacks her, cutting both her collarbone and ribs and puncturing one of her lungs. Finally near her wits' end and almost losing any hope of victory, Shinobu attempts to draw strength from replaying her older sister's words of encouragement in her head. Shinobu then prepares to strike at Doma again, despite her serious injuries and the immense pain she is in, and attacks him using Insect Breathing, Dance of the Centipede: Hundred-Legged Zigzag, to confuse him. She strikes Doma directly through his neck, impaling him and sending him flying towards the ceiling. Exhausted and wounded, Shinobu is triumphant upon seeing Doma being poisoned and seemingly dying from her attack.
However, Doma recovers almost instantly from her attack, by which Shinobu is further enraged. Now exhausted, fatally injured and dangerously near death, Shinobu is caught by Doma as they were fall down from the ceiling. He embraces her in a bear hug, and begins mocking her and calling her entire attempt to defeat him a waste of effort, assuring her she will live peacefully as a part of him, and mockingly asking her for her last words. She spitefully curses him as as he begins to kill her.
In the end, she is absorbed by the Upper Rank, just as her disciple Kanao Tsuyuri arrives at the scene, but just before her death she manages to signal to her Tsuguko with her hand. Her death causes Kanao to go berserk in order to try and save her, only to be repelled by an irritated Doma, who tells her not to interfere when he's in the middle of absorbing someone. He successfully devours Shinobu's body until her butterfly-shaped hair piece is the only thing left of her.
Doma notes aloud that Kanao must have been told something by the signal Shinobu gave her shortly before her death, wondering what she could have told her, as Kanao stands silently before him, enraged. As he licks Shinobu's hairpin, he says that it is a good night, as such fine feasts keep showing up one after another.
After Akaza's death, Doma says that he felt as if the Upper Rank had turned into a different creature before he died, and that his demise is inevitable because he never ate any women. Doma begins to cry over his 'best friend's' death, but Kanao just tells him to stop with the lies. She explains that she knows that every word that comes out of his mouth is nothing but random babble, and that it's impossible for him to be genuinely sad over Akaza because he can't feel anything at all. She says that every person born into the world has feelings, but Doma cannot understand these feelings and covers this up by spitting lies. He has never felt happiness, joy, pain and bitterness, and she says that he is nothing but an empty shell, and asks Doma why he is even born. Doma's face turns serious, and asks how Kanao can say something so cruel, to which the young Demon Slayer responds that she simply hates the Demon and wants to send him to hell this instant.
Suddenly, Doma appears behind Kanao, attempting to use one of his fans to cut her head off, but she quickly dodges and delivers a blow to his stomach, causing some of his organs to fall out. He doesn't seem to feel her blow, simply sending some of his ice lotuses at her and noting that her reaction speed has risen as she's able to adapt to his attacks. As he regenerates his body, he thinks to himself that Kanao could actually be stronger than Shinobu, a Hashira.
Trying to suppress her rage, Kanao attacks Doma with Flower Breathing: Fifth Form - Peonies of Futility, which he avoids with ease, smiling and commenting on how pretty her attack is before sending his Blood Demon Art: Barren Hanging Garden at her, which she barely fends off with Flower Breathing: Second Form - Honorable Shadow Plum. Doma notices that the main reason as to why she is able to counter and dodge his attacks is because of her immensely keen eyesight. Kanao herself is able to easily predict and guess his every move due to taking note of the subtle movements of his shoulders, gaze, toes, elbows and knees to predict what he'll do next and from their react and counter whatever attack he uses.Having figured out the reason to her reflexes and reactions,Doma decides that the best course of action to defeat her would be to simply take her eyes out with a swing of his fans. However Kanao successfully dodges his attack, and charges at him with Flower Breathing: Sixth Form - Whirling Peach which is nullified by Doma's next Blood Demon Art: Freezing Clouds. The Upper Rank sends two more attacks, Blood Demon Art: Lotus Vines and Cold White Princesses, her way, which she only just avoids before his Wintry Icicles begin to rain on her from above.
Kanao dodges again, moving farther away from the Demon, who laughs at her and says that she can't keep moving backwards if she wants to cut off his head. At a speed too quick for the eye to process, Doma runs at Kanao and snatches her blade, returning and planting it in the ground a distance away from her. He says she better come and get it quickly if she wants to survive him, before sending his Blood Demon Art: Scattering Lotuses at the Demon Slayer.
At that moment, Inosuke breaks through the ceiling of the room, protecting Kanao from Doma's attack with his Beast Breathing: Fifth Fang - Crazy Cutting. He looks at Doma's eyes and notices that he's Upper Rank 2, which he screams out, happily shouting that if he beats the Demon he'll become a Hashira and asking Kanao what his Hashira title should be. Inosuke notices Kanao's beaten-up face, and asks, surprised, what had happened, saying that Shinobu will get mad if she finds her Tsuguko in that state. Kanao remains silent, and the boar-headed Demon Slayer comes to a realization and asks her if Shinobu is dead, to which Doma replies that she isn't; she will live inside of him for all eternity because he has "saved" her from her pain and suffering. Inosuke recalls Shinobu's kindness to him, and vows to Doma that he'll kill him as he rushes towards the Upper Rank with his Beast Breathing: Fourth Fang - Slice 'n' Dice.
Kanao shouts out a warning to him not to breathe in Doma's cold air as the Demon remarks on how messed-up his technique is. Doma attempts to slice off Inosuke's head with one of his fans, but the Demon Slayer flips away from him, launching himself from the flat edge of the fan and landing next to Kanao. He hands her her blade, which he had taken back from Doma, and tells her never to let go of it again. Doma smiles and compliments Inosuke's speed as he jumps towards him, and the two swiftly engage in quick combat, launching Breath Styles and Blood Demon Arts at each other at alarming rates. Inosuke kicks Doma in the stomach, sending him skidding backwards as he dislocates the joints in his right arm and prepares to swing his sword at the Upper Rank's neck. He misjudges his swing and instead slices through Doma's eyes with his Beast Breathing: Ninth Fang - Extending Bendy Slash, and the Demon laughs at his unusual technique as his joints relocate.
Doma calls Inosuke messed-up and says that he's never met a kid like him before despite his long lifespan, to which the Demon Slayer replies that that's obvious, since he is the great Inosuke. At an incredible speed, Doma lurches forward and snatches his mask from his head as he's speaking, and Inosuke, enraged, yells at him to give it back. The Demon notices his face, and says he remembers Inosuke from somewhere, asking him if they'd met before. Inosuke responds that they hadn't as he would remember meeting someone like Doma, but the Upper Rank insists that they have, proceeding to stick his finger through his skull and into his brain to try and jog his memories.
Doma says that he knew Inosuke's mother, as his cult took her in, but Inosuke denies that he has a mother since he is raised by boars. Doma laughs and says that he must have a human mother since he's a human, and Inosuke shouts at him to quit spouting lies and give him back his boar mask, rushing in to attack the Demon again with Beast Breathing: Sixth Fang - Palisade Bite. His torso is slashed twice by Doma's fans, who smiles slyly and tells him not to interrupt before sending him flying back to Kanao with his ice.
Doma then 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. He explains that she met her demise by Doma's hands, who still believes he is saving her from her abusive household by eating her, but dropped her son off a cliff before the Demon's attack in the hope that he would survive. Memories of her begin to come back to Inosuke, who recalls the song she used to sing to him. Doma calls Kotoha stupid and unfortunate, saying that there isn't a happy moment in her life, and Inosuke, now enraged, thanks Doma for allowing him to remember this story and promises to not only to kill him for what he did to Kotoha and Shinobu, but to put him through hell beforehand.
Doma smiles again, and tells Inosuke that there is no heaven or hell in this world since they're nothing but fantasies created as a coping mechanism for weak humans. Inosuke shouts that he doesn't care if there's no hell because he can just make one himself, and tells Doma not to talk about his mother like she is unhappy.
The Demon sends his Freezing Clouds at Inosuke, which he blows away with Beast Breathing: Tenth Fang - Whirling Fangs as Kanao prepares to use another Breath Style to attack him from behind. Doma bats her away with one of his fans and moves away, and Inosuke yells at him to get back down so they can continue fighting. He replies that he can't, since Akaza has been defeated and he doesn't have time to play anymore, and uses his Blood Demon Art: Crystalline Divine Child to create smaller clones of himself for Inosuke and Kanao to fight instead. The clones attack the Demon Slayer duo with Doma's techniques as the real Doma starts to walk away, the clones using his Blood Demon Art: Scattering Lotuses, Frozen Lotus and Cold White Princesses attacks to hold the duo back. Now at the exit, Doma thinks about how Inosuke has managed to avoid breathing in the icy air from his techniques, and takes note of his sharp senses, saying he will make sure to remember him. He spectates their battle for a few seconds, observing how they use each technique so that he'll be able to kill them quickly the next time they fight, and then turns back towards the door.
Doma decides to create three more clones to let loose around the castle in order to kill the rest of the Demon Slayers there, and overhears Kanao shouting to Inosuke that they have to hold their ground just a little longer against the crystal children. He wonders what this could mean, but then dismisses it and begins to reach for the door handle.
Suddenly, Doma's face begins to melt. He drops to the floor, one of his arms snapping off, and his crystal children begin to crack and disintegrate as he wonders what's happening. Inosuke asks if it's a trap but Kanao, having been informed prior to the battle of Shinobu's plan to kill Doma, tells him that it isn't, and that the Demon is being melted by the poison Shinobu had in her body when he consumed her. She tells Inosuke to aim for his neck to finish him off, and, in a panic, Doma plays his final and most powerful card, Blood Demon Art: Rime — Water Lily Bodhisattva.
Doma's ice Bodhisattva statue slams down its hand on the floor of the room, creating a small earthquake that sends Inosuke and Kanao up into the air. Kanao notes that his technique lacks precision, and is clearly the Demon's last effort to protect himself with the little energy he had left, deciding to use her final form, Flower Breathing: Final Form - Equinoctial Vermilion Eye to try and break through him. Doma's statue grabs Inosuke in one of its hands as Kanao leaps towards the Upper Rank, hitting her blade against his neck. She is unable to cut it, so Inosuke, on the spot, comes up with and uses Beast Breathing: Sudden Throwing Strike to hurl his blades at Kanao's to give it the extra strength needed to cut Doma's neck. They succeed in beheading him, and Upper Rank 2 and his statue begin to disintegrate.
As his head falls to the floor, Doma thinks about how pathetic he must be to have lost to a poisoner like Shinobu, someone who didn't even have the strength to kill a Demon by cutting its head. He thinks to himself that he shouldn't give up, and that he should try and regenerate his head like Muzan can and Akaza almost did, but soon comes to realize that that would be futile, and that he is going to die. In his final moments, Doma says that he is not afraid of death or even frustrated that he lost, and recalls his past as a human and his inability to feel any emotion.
Now in the afterlife, Doma opens his eyes to Shinobu, who is holding his severed head in one hand. She asks if he's finally dead, and says that she can now rest in peace. Doma asks her if her name is Shinobu or Kanae, and she says that it doesn't really matter because he makes her sick and she doesn't want him to say her name anyway. He ignores this, going on to compliment her for her poisoning skills and express his surprise at not noticing it had circulated through him before it is too late, and she smiles and explains that a Demon called Tamayo helped her make it. Shinobu says she is frustrated that she couldn't kill him herself and the fact that she can no longer help the other Demon Slayers defeat Muzan, but that she is still satisfied with the result. Doma gives her a look, and Shinobu asks what's wrong, to which the Demon says that he feels like his heart is beating and wonders if he is in love with Shinobu. He thinks aloud that if love really exists, heaven and hell must do too, and asks Shinobu if he wants to go to hell together with him. She tells him to go to hell, and calls him a worthless bastard.