The Swamp Demon was a pale skin 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.
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 their 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 Kibutsuji who gave him a sound beating and transformed him into a Demon. Kibutsuji warned him to never say anything about Kibutsuji to others and that he would instantly know if he ever did. The Swamp Demon was left permanently terrified of Kibutsuji.
The Swamp Demon using his ability to travel in his own pocket dimension swamp would perform multiple kidnappings of girls at night where he would drag them into his swamp to eat them. Kazumi's fiancee Satoko would be one of his victims, prompting action from Tanjiro Kamado to slay him.
Tanjiro would find a distraught Kazumi who told him of what happened even though nobody believed him. Tanjiro believe him and using his sense of smell would be able to discern the Swamp Demon and interrupt his next act. The Swamp Demon was frustrated that Tanjiro would get in his way and tried to kill him by using his swamp abilities along with his split selves to overwhelm him, but Tanjiro is able to defend himself.
When Kazumi asks if Satoko is alive, the Swamp Demon doesn't recognize the name but reveals his stash of belongings, showing Satoko's ribbon, proving on the demon's words that he ate her, leaving Kazumi in despair.
When the Swamp Demon tries to take advantage of an opening to kill Tanjiro, Nezuko Kamado, repels him with a kick on his face from inside the box and joins her brother in fighting the Swamp Demon. As a swamp portal opens before Tanjiro he entrusts Nezuko with protecting Kazumi and the potential victim while he fights the demon in his territory. 
The Swamp Demon is confident he can win in his area, without realizing that Tanjiro's lung capacity was heavily fortified under the harsh training he underwent. When two of the Demon's selves tried to pincer attack Tanjiro he uses the Breath of Water's Sixth Style, Whirlpool to perform a rotation slash that eviscerated the two demons, killing them.
The remaining demon was left to fight Nezuko whom he had difficulty to deal with due to her immense strength but slowly got used to her attacks and tried to decapitate her, but Tanjiro escapes the swamp and slices the demon's arms, leaving him crippled and cornered.
Tanjiro asked why he attacked girls like he did, and he responded that he did them a favour by eating them before time passed and they became undesirable. 
Angered by this, Tanjiro sliced his mouth enough to hurt him but keep his talking ability intact. Tanjiro pressed for information about Kibutsuji, and the Swamp Demon was left terrified and muttering that he couldn't say anything. Tanjiro could sense that the demon's fear was genuine. The Swamp Demon in the apex of his horror, regenerates his arms and tries a final assault on Tanjiro who easily evades his attack and decapitates him, thus ending his hunting spree for good.
Abilities and Powers
The Swamp Demon was a modestly powerful demon who could use Blood Techniques to his advantage.
- Swamp Space: 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 Split: 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