The Swamp Demon was a pale skinned young man with long blue hair with two long strands of violet on both sides of the face, supported by two golden hair separators. As usual in several Demons, he had claw-like nails and the veins on his forehead are quite prominent, in his case, to the point of replacing his eyebrows. His eyes were red without pupils and with prominent dark eye bags. Because he could be divided into two Demons apart from himself, each demon differed from the other in the number of horns and clothing. The three Demons wore shinobi clothing.
The main body had a single horn in the upper center of its forehead and wore a full-length long-sleeved shinobi uniform with meshes on its wrists and neck, the mesh of the neck having three small ribbons centered hanging from it. There were divisions inside his uniform coat where he could keep the collection of hairpins of his victims.
The second body had two horns, one on each side of its forehead. His shinobi attire did not have sleeves, instead, it had two small triangular meshes in the armpit area and a large triangular mesh in the chest area.
The third body had three horns, each in the position in which the other two bodies had their own horns and wore a shinobi garment sewn with meshes on both sides of the arms, shoulders, and thorax. Under his coat, he wore a sleeveless bottleneck shirt.
By dividing into three, the Swamp Demon also divided certain aspects of his personality to the point that each behaved differently from the other.
The main Demon was calm, analytical and serene, gave advice to others and even suggested to his second body to calm down, which is why he was seen as the leader of the trio of demons. He was someone with logical thinking and proof of this is seen when he questioned Tanjiro's relationship with Nezuko, which he sees as abnormal due to the behavior of demons with humans, he is also someone who prematurely celebrated the victories by telling Tanjiro that it was a idiot for entering his swamp voluntarily. He rarely lost his calm. He also displayed certain sociopathic behavior, targeting girls who fit a type (beautiful girls of age 16) and collecting trophies of doing so, in this case, the girls hair ornaments.
The second Demon, despite having the same level of intelligence as the principal, was more violent, impulsive and emotional, it was even easy to alter it. It showed the emotions that the principal easily hid.
The third, however, unlike the others, had no visible emotions and acted more on instinct to the point of being similar to a hunting dog accompanying his hunting master, following both the main and the second. He could not even articulate a word.
The traits shared by the three are their love to kidnap and eat 16-year-old girls, their excellent teamwork and the deep fear they had for Muzan Kibutsuji to the point that their bodies trembled at the sound of his name. They believed that after 16 years old, women would age, they would become ugly and their flesh would taste bad, and they even thought that they should be grateful to be devoured at that age.
When the Swamp Demon was a human, he encountered Muzan who gave him a sound beating and transformed him into a Demon. Muzan warned him to never say anything about Muzan to others and that he would instantly know if he ever did. The Swamp Demon was left permanently terrified of Muzan.
Every night in the town to the northwest, another young girl disappears. One night, Kazumi and his fiancée, Satoko, walk through town and suddenly the girl goes missing. He's blamed for her sudden disappearance, but rumors swirl that there might be a Demon lurking at night.
Tanjiro of the Demon Slayer Corps receives his first mission to head into town and hunt that Demon down. He believes Kazumi's story and the pair spend all day searching for clues. It is not until that night when the Demon becomes active again.
The Demon uses his Blood Demon Art to drag a a girl named Tokie into his swamp dimension. Its scent gets stronger, so Tanjiro is able to track it down to the exact spot the Demon is holding her underground. He stabs his blade into the ground, forcing the pocket portal to open. It spews out murky water, but Tanjiro sees the Tokie's kimono and is able to pull her out.
A morphed Demon appears from the bog and screeches at Tanjiro before disappearing again. Kazumi catches up to him late and Tanjiro asks him to hold Tokie so he can protect them. Tanjiro examines the area and the possible places the Demon could appear from. Even when submerged in its own swamp, the Demon's scent isn't erased.
Tanjiro can smell the Demon ready to attack, he prepares the Water Breathing: Fifth Form - Blessed Rain after the Drought as a counter and rushes to the spot where the portal will appear. However, three bog puddles appear and three different Demons sprawl out of the swamp. The young Demon Slayer jumps to avoid their grasp and calms himself as he switches forms. Tanjiro tries to lay waste to the three Demons using Water Breathing: Eighth Form - Waterfall Basin.
Unfortunately, because Tanjiro hastily switched forms, his attack was too shallow of a strike and he only manages to leave off the Demon's arms. He calms himself again, determined to make the Demon talk about Muzan Kibutsuji. The Demon tries to go for Kazumi and Tokie, but Tanjiro protects them with Water Breathing: Second Form - Water Wheel.
Once again, Tanjiro's attack is too shallow, but he was able to save the others. He takes two more quick swings at the demon and misses, forcing the Demon to retreat. The young Demon Slayer can't chase the Demon or take a full swing with Kazumi and the girl nearby. The two-horned Demon isn't amused and demands the Demon Slayer back off or else the girl will lose flavor. The three-horned Demon tells his other self to calm down and claims to have eaten plenty of sixteen-year-old girls already. Kazumi demands to know what the Demon did with his fiancée, only to find out that they devoured her.
Enraged, Tanjiro quickly evades the three-horned Demon's surprise attack. Its arm cracks the wall next to Tanjiro but he jumps above the Demon and slashes its arm off. He considers this a miss and then realizes he's come too close to the wall. Tanjiro twists his body over to avoid the attack and is forced to back up by an attack from underground. The two-horned Demon nearly lands a hit by appearing from behind, but Nezuko kicks out of her box and breaks the Demon's neck.
The Swamp Demon questions why a swordsman would be accompanying a Demon. The broken two-horned self fixes its neck and they all fade back into the bog. Under hypnotic suggestion from Sakonji Urokodaki, Nezuko sees all humans as her family, and she will fight to protect them. She rushes the Demons as they appear from the swamp and tries to crush them with a fierce axe kick.
The Demon disappears into the bog in time to evade her attack. They try to lead the girl away but Tanjiro asks her to return to his side. She runs back to him and one of the Demons try to grab her, but she easily flips over him to avoid it. This allows Tanjiro to realize that he can let Nezuko protect Kazumi while he focuses on attacking.
A swamp portal opens beneath them as a distraction while the two-horned Demon goes after the girls. Nezuko protects Kazumi and keeps the Demon back. Tanjiro asks Nezuko to continue protecting Kazumi while the young Demon Slayer heads underground. He descends into the Demon's bog dimension, where there is hardly any air and the clothes of many victims float about.
The two Demons inside the swamp believe it must be hard for Tanjiro to move or breathe, and that he's a fool for diving in. They swim at him at high-speed, but the young demon slayer isn't to be underestimated. On top of Sagiri Mountain, where Tanjiro spent over two years training, the air was much thinner than inside the swamp. There, Tanjiro also learned Water Breathing forms that can only unleash full potential underwater.
The Demons dart away at a sharp angle just as Tanjiro is about to counterattack, but he knows he can just wait for them to get close again. The Demons try and rush him from a different angle, but Tanjiro can smell the opening threat. He uses a technique that can be unleashed with no sure footing, a fierce slicing vortex created by a violent twist of his upper and lower body; Water Breathing: Sixth Form - Whirlpool.
The slicing whirlpool slashes apart the Demons into pieces, killing them instantly. Despite his victory, Tanjiro is saddened by the number of victims that the Demons have claimed. The boy is running short on breath and swims back toward the entrance to the swamp. Meanwhile, Nezuko is overpowering the last remaining Demon with her bare hands.
Nezuko weaves through four of the Demon's strikes in a row, including a strong hammer fist that crushes the ground. She follows up with a powerful jab into the Demon's gut that's strong enough to make him cough up blood. The Demon manages to evade her next punch, but he immediately gets clobbered by another punch and kick combo. The Demon tries to slip back into the bog, but Nezuko acts quickly and sweep kicks so the Demon can't escape. She tries to hit him with a flying kick but he's able to block it.
Nezuko is too fast for the Demon to dive back into the swamp, but he gets used to her straightforward fighting style. They exchange a few blows that they're able to either redirect or avoid until the Demon narrowly blocks another strong to his head. He's able to block her next combo, confident he can regenerate from anything she throws at him. The Demon rushes Nezuko and is able to block her jab, giving him an opening to slash her forehead with his claws.
The Swamp Demon tries to finish off Nezuko by stabbing his claws into her face, but Tanjiro emerges from the portal in time to save his sister. Tanjiro cuts off the Demon's arm, demanding the Demon not lay a hand on his beloved sister. He follows up by slicing off the Demon's other arm before it can attack again, ultimately defeating the Demon.
The last remaining Swamp Demon falls over and realizes its other selves have been killed. Tanjiro tells him he gives off a terrible stench and asks how many girls he's kidnapped. The Demon tries to defend its actions by claiming he was killing girls so they wouldn't get old and taste bad. Irritated, Tanjiro cuts out the Demon's tongue and moves on to the next question.
Tanjiro presses his blade close to the Demon and forces him back against a fence. He demands to know all the information the demon has on Muzan. However, Tanjiro is not prepared for the crippling fear of Muzan which the Demon has been instilled with. He cowers and trembles down to his bones, claiming he can't say anything.
The scent of fear is extremely strong and the Demon acts out of panic. He grows his arms back and lashes out, forcing Tanjiro to cut his head off and end him.
The Swamp Demon was a modestly powerful Demon who could use Blood Demon Arts to his advantage. Just as Kyogai is similare to Nakime, the Swamp Demon is similar to Hantengu, what with the personality splitting ability.
Swamp Domain: The Demon's namesake ability. He is able to conjure a pocket swamp dimension in which he can navigate freely. The "swamp" often manifests as a puddle that travels on the ground which can be used to infiltrate households and perform kidnappings with nobody realizing. Although the puddle looks small enough to fit one person, its interior is far larger and has the properties of an actual swamp. The Swamp Demon can put this swamp to his advantage by attacking targets using his great swimming ability.
Self-Replication: The Swamp Demon can split himself into three selves, distinguished by their clothing and the number of horns on their heads (from 1 up to 3). All the three Demons can travel through their own puddles and overwhelm any target with number advantage.
- The Swamp Demon was ranked in 51st place as of the first character popularity poll with 8 votes.
- The Swamp Demon's ability to divide his emotions/sense of self into different bodies is similar to the Blood Demon Arts of Upper Rank 4, Hantengu.
- Based on his attire and stealth like Blood Demon Arts, it's likely the Swamp Demon was previously a Shinobi not unlike the Sound Pillar, Tengen Uzui.