Muichiro Tokito/Synopsis

History
Muichiro and his older twin brother, Yuichiro, were born to a woodcutter and his wife. When they were 10, their mother caught a cold which developed into bronchitis and killed her. Their father, who had gone to fetch medicinal herbs for their mother during a storm, ended up falling off a cliff to his death.

The two orphaned siblings survived together, with Muichiro taking after the emotional and kind nature of their father. Yuichiro instead held a cold and impassive outlook on life.

In the following spring, Oyakata's wife, Amane Ubuyashiki, arrived to recruit the twins for the Demon Slayer Corps. Muichiro's excitement is opposed by Yuichiro's unwelcoming and rejecting reception. In an outburst, Yuichiro revealed that he blamed the death of their parents on their hardworking and selfless nature and adopted a cold outlook, hoping to keep himself and his brother alive.

While Amane continued to visit the twins, Yuichiro chased her away each time. In the following summer, a Demon arrived at their cabin during the night and attacked, resulting Yuichiro losing his left arm while trying to protect Muichiro. As the Demon badgered them for their worthlessness, Muichiro lost himself in a rage and destroyed the Demon with all the tools at their disposal and wooden logs.

By sunrise, the Demon withered into ash and Muichiro dragged himself back to the house. There, he found Yuichiro near death praying to the heavens that his brother might survive. He laid there grasping his brother's hand as he passed away. Amane and her daughters later arrived to heal Muichiro.

Afterwards, Muichiro would lose his memories and leave his old life behind to serve the Demon Slayer Corps, and, due to his strength, he quickly rose to the rank of a Hashira. With his memories gone, he began taking after Yuichiro's personality. He also met Kyojuro Rengoku during this period in the Demon Slayer Corps, who told him they should do the best they could as fellow Hashiras.

Rehabilitation Training Arc
After Tanjiro Kamado is brought to the Hashiras' meeting for interrogation, Muichiro is present and later introduced as the Mist Hashira. When questioned on how he felt about Nezuko Kamado being kept alive, he retorted that he did not mind and would forget about the circumstance anyways.

When Tanjiro suddenly interrupts Kagaya in the middle of his meeting with the Hashiras, Muichiro flicks him in the head with a pebble and advises him to not interrupt their master, telling him to leave.

Mugen Train Arc
Muichiro is seen receiving the news from his Kasugaigarasu about Kyojuro's death.

Entertainment District Arc
While Tengen Uzui is battling against Gyutaro, Gyutaro compliments him on his supposed natural talent for fighting. Tengen denies this and instead thinks of Muichiro and Gyomei Himejima as individuals with exceptional aptitude towards swordsmanship.

Swordsmith Village Arc
Muichiro first appears in the Swordsmith Village when he is seen arguing with Kotetsu. He tries to convince the latter to provide him with the key to Yoriichi Type Zero, slapping him when he refuses to comply. Tanjiro intervenes, grasping his arm and attempting to crush it like he did to Genya Shinazugawa after the Final Selection. However, to his surprise, Muichiro does not budge despite having a smaller frame and thinner arms, instead punching Tanjiro in the stomach and reprimanding him for his weakness. He sees the box on his back and reaches for it, sensing a demon, but Tanjiro slaps his hand and tells him not to touch it while rescuing Kotetsu from his grasp. Kotetsu continues to refuse handing over the key even under torture, and Muichiro chides him for his talk, stating how he is wasting a Hashira's time and that swordsmiths are only good for making weapons. Tanjiro interrupts, angry at Muichiro's inconsiderate attitude, but he gets knocked out by him.

After Tanjiro regains consciousness, he goes with Kotetsu to watch Muichiro fight against the mechanical doll, witnessing him breaking it. Muichiro remarks about the useful training and takes one of Type Zero's swords, asking Tanjiro to dispose of his broken one. Tanjiro doesn't sense any malice coming from him, but he is still upset about his attitude.

Later that week, Muichiro pinches Tanjiro to wake him up, inquiring him about the whereabouts of Kozo Kanamori, his new swordsmith. When Tanjiro offers to go with Muichiro to find him, the latter questions why he would do so. Tanjiro replies that helping others could end up positively affecting himself, leading to Muichiro suddenly feel a sense of familiarity and he questions Tanjiro about what he just said. However, before he can do so, Nezuko awakens and breaks the flow of their conversation, Muichiro wondering where he saw her before. Their conversation is suddenly interrupted by the entrance of Upper Kizuki Four, Hantengu.

Muichiro immediately attacks Hantengu using Mist Breathing, Fourth Form: Shifting Flow Slash. Hantengu dodges and he comments on his speed, attacking him again after Tanjiro and cutting off his head. Tanjiro, remembering the situation with Daki and Gyutaro, yells at Muichiro to keep focusing as Hantengu splits into two bodies, Sekido and Karaku. Karaku attacks Muichiro and blasts him out of the building and into the forest.

As he rushes to return to the village, Muichiro sees Kotetsu being attacked by a demon. He contemplates saving him but suddenly remembers Tanjiro's words and beheads the demon, telling Kotetsu to escape. When Kotetsu intensely thanks him and apologizes for his original hatred towards him, Muichiro's memories spur as he remembers recovering under the care of the Ubuyashiki family.

Kotetsu leads Muichiro to where Kozo is. As Kozo starts making for the shed to retrieve the new blade, Muichiro pulls him back as he senses the presence of Upper Kizuki Five, Gyokko. Muichiro attacks him, but Gyokko moves to different pots and avoids his slashes. He summons more fish demons that send an array of needles towards Kozo and Kotetsu, but Muichiro shields them and tells them to move out of the way. Gyokko reveals the needles are poisonous and berates Muichiro, resulting in another sudden memory recurring to him. He tries to attack Gyokko, but gets trapped in his Blood Demon Art, Water Prison Pot, his attempt to break out using Mist Breathing, First Form: Low Clouds, Distant Haze unsuccessful. As he thinks about his approaching demise, he suddenly hears his father speaking from Tanjiro's body. Kotetsu swiftly arrives to provide him with a breath of air and he is able to break out using Mist Breathing, Second Form: Eight-Layered Mist, remembering it is his father who said the same thing as Tanjiro had earlier.

After escaping from the water prison, Muichiro's memories of his past all come back. The anger over his brother's death manifests into his Demon Slayer Mark, his speed and abilities dramatically increasing as he goes into the shed and attacks Gyokko, who comments on his relaxed face. Gyokko retaliates with his Blood Demon Art, Octopus Vase Hell, and Muichiro's blade breaks trying to cut it, prompting Kozo to quickly hand him his new one. Muichiro slices through the demon's attack and thanks Kozo, in which he replies that he only followed the instructions of the late Tetsuido. At the mention of his name, Muichiro remembers his old swordsmith's worry for him and internally apologizes, remarking that he is okay now and uses Mist Breathing, Fifth Form: Sea of Clouds and Haze, landing a blow on Gyokko's neck.

Muichiro and Gyokko start insulting each other, with the latter growing furious after the former insults his pot. He uses another Blood Demon Art, Thousand Needle Fish Kill, but Muichiro easily counters it with Mist Breathing, Sixth Form: Lunar Dispersing Mist and Mist Breathing, Third Form: Scattering Mist Splash, slicing the head of the demon's molt as he escapes to a tree. When Gyokko tries to show off his new form, Muichiro's indifferent attitude towards it enrages him again and he attacks, which Muichiro dodges and smiles, commenting on the uselessness of his offense.

Another scuffle between the two ensues, with Gyokko using his Blood Demon Art, Killer Fish Scales and Muichiro responding with the form he personally developed: Mist Breathing, Seventh Form: Obscuring Clouds. When Gyokko tries to find him in a sea of mist, Muichiro effortlessly decapitates him.

In the aftermath of the battle, Muichiro starts foaming at the mouth due to poison and falls unconscious after verifying Kotetsu is alive, his family congratulating him on a job well done in his consciousness.

When Tanjiro is running after Hantengu, Muichiro throws Hotaru Haganezuka's unfinished Nichirin Blade that is retrieved from Type Zero, granting Tanjiro the ability to defeat him.

After the battle ends, Tanjiro and Muichiro thank each other, Muichiro for the sword and Tanjiro for helping him regain his memories. Mitsuri Kanroji later pulls them all into a group hug.