Slasher

The Slasher (りき) was the nickname given to a demon that appeared during the investigation of the Mugen Train and was responsible for a number of human attacks. He was defeated by Kyojuro Rengoku.

Appearance
Slasher was a bald middle-aged man of a fairly muscular build and very pale skin. His eyes had a lime green iris, a black sclera and vertical slits for pupils, and he had sharp pointed nails that are dark blue in color and several prominent fangs in his mouth. Most prominently, Slasher's whole body was covered in dark blue tattoos, running across various places on his face, limbs and torso. These tattoos light up and glow blue when he uses his Blood Demon Art. He also wore a sleeveless blue shirt and black leggings complete with tabi socks and a pair of zōri with blue straps. Slasher was usually seen with a maniacal expression on his face.

Personality
Like most demons, Slasher was a ruthless individual that only seems to care about murdering and eating humans, He also was shown having a sadistic and malicious side, off-handedly mentioning he slashed a woman till he became bored and when he dug his claws into a young boy. He was also shown to be quite arrogant at times, looking down on the Flame Hashira, Kyojuro Rengoku and boasting his speed in front of him as well as challenging him to a duel twice. However, Slasher was not above using underhanded tactics like taking a hostage to win against Kyojuro.

Slasher was also rather petty, trying to find whatever means to provoke Kyojuro and growing further frustrated and more aggressive when the young man both held his ground, retorted back and even stopped his murders.

Mugen Train Arc
Slasher encountered and terrorized a village in search of prey. He found a young woman roaming around at night but thought she didn't seem worth eating. Instead, he horrifically clawed her body till he became bored and left her to die. The next night, he boarded a train but found it empty, apart from one hapless train conductor. He quickly killed him but found him not worthy to devour too and left his body to be discovered. The news of his actions frightened the village and people feared leaving their houses after dark, causing business usually active at night to halt. The Demon Slayer Corps subsequently sent Kyojuro Rengoku to investigate.

Slasher happened upon a maintenance facility for trains in search of prey. Unbeknownst to him, the facility held the Mugen Train, which was assumed to hold a demon likely responsible for numerous disappearances reported. He found a young worker and grabbed him in his claws. The scene drew the attention of Kyojuro, investigating the Mugen Train, and Slasher was confronted by the Hashira. While facing him, he wretches suddenly, before stepping on a bento box and claiming that human food is disgusting to him now as a demon. He taunted Kyojuro into attacking him, using his Blood Demon Art to rush around the Hashira and goad him into attacking, warning him that the young boy will be dead if he directly tried to attack. Kyojuro acknowledges his speed but tells him not to get cocky. Slasher then mentioned how he injured a woman the previous night but was told by Kyojuro that she will recover completely.

Angered, he noticed the stepped bento box and Kyojuro's angered expression, before realizing the box likely came from a nearby station. Deciding he will kill the vendors to spite Kyojuro, he first tried to kill the boy he was holding but has both of his hands cut off. He retreated and steps outside the facility to regrow both his hands, before smelling the air for the bento vendors' scent and following the tracks, using his Blood Demon Art to cross the great distance in minutes. He arrived at the station and entered, seeing the young girl setting up her bento stand. As she staggered in fear of seeing him, Slasher moved forward to kill her but was hit by a bento box from behind, thrown by her grandmother. Momentarily distracted by the smell, he soon caught up to her outside and tried to crush her throat. Kyojuro thwarts his attack and slices his left achilles, saving her.

Slasher then turned his attention to her grandmother instead but was confronted by Kyojuro once more. Wondering aloud how he manage to catch up to him so quickly, Kyojuro repeated his earlier words not to get cocky facing him. Slasher decided to make a contest for the two to determine who is faster, if Kyojuro can kill him before he kill the girl's grandmother. To this, Kyojuro mocked him for being slow and rushed forward, decapitating Slasher before he was able to react, ending his short spree of terror.

Abilities
Overall Abilities: Slasher was a decently powerful demon, as evidenced by developing a Blood Demon Art and displaying skill in utilizing it for combat. He proved mildly troublesome for even a Hashira to handle, albeit most of it attributed to his penchant to use human shields, but in terms of direct combat, Slasher proved no match for the Flame Hashira.

Enhanced Regeneration: Like all demons, Slasher possesses enhanced regeneration. He was able to grow both his hands back in moments after having them cut, as well as healing his sliced Achilles tendon just as quickly.

Blood Demon Art
Augmented Speed: Slasher's Blood Demon Art enables him to greatly augment his own speed, allowing him to traverse great distances in a short amount of time. This technique is activated whenever his dark blue tattoo glows. The speed at which he moves is such that his entire figure is covered in a blue light.

Trivia

 * Slasher is an entirely anime-original character and does not appear in the manga at all.