Kotetsu

Kotetsu lives in the hidden Swordsmith Village, where Nichirin Blades for the Demon Slayer Corps are forged.

He is the current owner of the ancient combat training puppet Yoriichi Type Zero, created in the Sengoku era by his ancestor.

He works as an apprentice swordsmith, but his true specialty is maintaining and operating Yoriichi Type Zero.

Because of his lineage and the doll’s importance, he shoulders heavy expectations despite still being a child.

Kotetsu has an extremely blunt, abrasive personality and is stubborn to the core.

He holds grudges for a long time and, once he decides on something, tends to charge ahead and drag everyone around him into chaos.

Despite his rough attitude, he possesses keen analytical ability and a sharp eye for both craftsmanship and swordsmanship.

This high awareness also leads him to clearly see his own limits, which causes him to fall into early discouragement and self‑doubt.

His face under the mask is almost identical to his mask, except that his mouth and cheeks are visible, with red speckled marks on his cheeks.

Because of this resemblance, other characters even question whether he needs the mask at all.

Kotetsu is a hardline, foul‑mouthed kid who has no hesitation in criticizing adults, Demon Slayers, or anyone else.

He frequently fires off lines such as “If you die from something this minor, you’re trash! Do your best!”

He is fiercely proud of Yoriichi Type Zero and of his craft.

When someone disrespects the doll or treats it as expendable, he explodes with anger and digs his heels in.

Underneath his stubbornness, he genuinely cares about people who put their lives on the line, such as Tanjiro Kamado and Muichiro Tokito.

He wants them to survive, and his harshness often comes from wanting them to become strong enough not to die.

Kotetsu’s analysis skills are outstanding: he quickly spots strengths, weaknesses, and habits in others’ sword techniques.

However, his teaching style is crude and unrefined; even Tanjiro calls his sword instruction “amateur level.”

He is also naive in some areas because he is only ten.

This leads him to set up inhuman training regimens without fully understanding how dangerous they are.

Kotetsu’s main talents lie in mechanical analysis, observation, and operation of Yoriichi Type Zero.

He understands the doll’s structure, hidden functions, and limits better than anyone in his generation.

He can accurately read combat habits in a single glance, as he does with Tanjiro’s tendency to move on reflex without proper foundational decision‑making.

He then builds training around eliminating those weaknesses, even if his methods are brutal.

As a swordsmith apprentice, he has a solid foundation in production and repair, though he believes his talent does not measure up to his ancestors.

His self‑awareness about his own skill level contributes to his early desire to give up.

In spinoff school settings, he shows high intelligence and mechanical understanding, such as building multiple robots and mastering crane game mechanics.

There, his talent for machine design and probability stands out more directly.

Encounter with Muichiro Tokito and Tanjiro Kamado

Kotetsu is first involved in the story when Muichiro Tokito comes to use Yoriichi Type Zero for combat training.

Kotetsu refuses because the doll is already near its structural limit after years of use and is on the verge of breaking.

Muichiro, acting purely on cold logic and without malice, tries to force the matter with physical intimidation.

Kotetsu resists, leading to a confrontation where Tanjiro Kamado steps in to defend him.

Due to the massive difference in ability, Tanjiro is quickly overpowered by Muichiro.

Realizing he cannot protect the doll by force, Kotetsu reluctantly hands over the key needed to activate Yoriichi Type Zero.

Decision to Train Tanjiro with Yoriichi Type Zero

After Muichiro’s training session, Yoriichi Type Zero’s arm is destroyed, which enrages Kotetsu.

He confirms the doll can still move and decides to secretly use functions he had kept hidden from Muichiro in order to train Tanjiro.

Kotetsu harshly critiques Tanjiro:

He points out that Tanjiro moves out of habit instead of fully reading his opponent and making calculated choices.

He declares that Tanjiro’s fundamentals are inadequate and that it is a miracle he has survived this long in the Demon Slayer Corps.

He then vows to thoroughly crush Tanjiro’s weaknesses and refuses to give him food until he can meet Kotetsu’s demands.

This marks the beginning of an extremely severe training regimen.

Extreme Training Regimen

Because Kotetsu lacks proper knowledge of safe training methods and human limits, his regimen becomes dangerously extreme.

He demands prolonged training with virtually no food, no water, and no sleep, pushing Tanjiro beyond normal endurance.

Over seven days of this punishment, Tanjiro fights Yoriichi Type Zero to the point of hallucinating.

At one point he sees himself crossing the bridge over the Sanzu River (a death‑like near‑experience), which leads him to perceive the “sense of death” in battle.

That near‑death sensation helps Tanjiro develop the ability to detect killing intent and openings in an opponent, allowing him to strike at precise intervals.

Kotetsu’s reckless but relentless regimen, combined with Tanjiro’s own will, results in a significant breakthrough in Tanjiro’s technique.

Kotetsu recognizes that Tanjiro is holding back to avoid damaging the doll, which is a reflection of Tanjiro’s kindness.

Struck by this, Kotetsu both criticizes Tanjiro for being “too soft‑hearted” and simultaneously begins to truly wish for his survival.

Discovery of the Hidden Sword

In the climax of the training, Tanjiro finally destroys Yoriichi Type Zero.

From within the broken doll, they discover a single old blade hidden inside its structure.

Kotetsu decides to give this ancient sword to Tanjiro, treating it as a reward and a legacy.

However, Hotaru Haganezuka appears, declaring “Leave it to me!” and tries to seize the sword outright.

A chaotic tug‑of‑war erupts as Kotetsu desperately fights to keep the blade from being stolen.

With the mediation of Kozo Kanamori, they eventually resolve the conflict: Haganezuka will restore the blade using his clan’s secret polishing techniques, and Tanjiro will be its wielder.

Upper Rank Attack on the Swordsmith Village

During the assault on the Swordsmith Village by Upper Rank demons, Kotetsu shelters with Kozo Kanamori at Hotaru Haganezuka’s workshop.

In the forested mountains, they encounter one of Gyokko’s fish‑like minion demons.

Kotetsu is nearly killed but is saved just in time by Muichiro Tokito.

After regrouping with Kanamori, Kotetsu heads back to Haganezuka’s workshop in order to help protect him while he continues polishing Tanjiro’s new blade.

At the workshop, Gyokko traps Muichiro in his Water Prison Pot, attempting to drown him.

When Gyokko leaves the area momentarily, Kotetsu seizes the chance to rescue Muichiro.

Despite being extremely outmatched and still just a child, Kotetsu attacks the Water Prison Pot to break it.

In the process, he is ambushed from behind by one of Gyokko’s fish demons with bladed fins and is stabbed in the pit of his stomach, suffering a serious injury.

Even so, he refuses to give up and continues trying to save Muichiro.

He breathes air into the Water Prison Pot to supply Muichiro with oxygen, keeping him alive long enough to act.

Thanks to Kotetsu’s desperate assistance, Muichiro manages to use Mist Breathing Second Form: Eight‑Layered Mist to shatter the Water Prison Pot.

This act, combined with earlier words from Tanjiro, helps restore Muichiro’s lost memories.

Later, when Muichiro has defeated Gyokko and collapsed from exhaustion, Kozo Kanamori panics over his condition.

Kotetsu appears again, badly injured and bleeding heavily from his arm but still alive.

He had tucked Kyojuro Rengoku’s keepsake sword guard, which Tanjiro had entrusted to him, inside his clothing, and this object protected him from what could have been a fatal wound.

Muichiro’s decision to save Kotetsu earlier ultimately contributes to victory and survival for everyone involved.

Kotetsu is known for his cutting, over‑the‑top remarks that mix cruelty with a strange form of encouragement.

A representative line is:

“If you die from something this minor, you’re trash! Do your best!”

During training with Tanjiro, he lectures him at length:

He tells Tanjiro that he is moving by habit and not by properly reading his opponent.

He insists that Tanjiro’s basics are not properly grounded and that it is a miracle he is still alive in the Demon Slayer Corps.

He also threatens:

He will not give Tanjiro any food until he can execute what Kotetsu demands of him in training.

In the anime preview segments, he continues this pattern, firing off long strings of idioms and nags about not slacking off just because training went well once.

His relentless commentary even makes Tanjiro try to run away with Nezuko Kamado at one point.

In the animated adaptation’s Swordsmith Village Arc, Kotetsu appears as a supporting but memorable character.

He first appears when Muichiro Tokito demands to use Yoriichi Type Zero and Tanjiro intervenes.

The anime highlights Kotetsu’s shrill voice, frantic movements, and absolute emotional honesty.

His tough‑love relationship with Tanjiro and his grudging softening toward Muichiro receive extra emphasis.

In one next‑episode preview, Kotetsu launches into an intense “training announcement,” warning Tanjiro not to slack off and throwing out proverb after proverb.

His barrage of harsh words becomes so overbearing that Tanjiro and Nezuko attempt to flee the training.

In another preview, he carelessly says something extra that enrages Hotaru Haganezuka.

This results in a comedic chase reminiscent of Haganezuka’s earlier rampage after Tanjiro in the Entertainment District Arc.

In an anime‑original final episode scene, Kotetsu once again says one unnecessary remark too many to a not‑so‑honest‑about‑his‑feelings Haganezuka.

Haganezuka loses his temper and physically swings Kotetsu around, reinforcing the running gag that Kotetsu’s mouth gets him into trouble.

Kotetsu does not appear in the original Kimetsu Academy shorts attached to the main anime.

However, he features in the separate manga‑style spinoff “Kimetsu Academy!” serialized in a children’s magazine.

There, he is a fourth‑grade student in the elementary division of Kimetsu Academy.

He helps out at Hio PIZZA (Hiotto‑ko Pizza), a pizza place run by Tecchin Tecchikawahara, Kozo Kanamori, and others.

In this setting, Kotetsu is treated as a relative of Tecchin Tecchikawahara.

The magazine release initially called him the child of the pizza shop’s owners, but the later collected volume changed this to say the owners are his relatives.

Kotetsu is close friends with the Tokito brothers, Muichiro Tokito and Yuichiro Tokito, who are portrayed as genius kids.

Fittingly, Kotetsu himself is depicted as very smart, especially in machine‑related fields.

In Kimetsu Academy, Yoriichi Type Zero appears as a robot built by Kotetsu rather than an ancient doll.

He has constructed at least two units, which can appear simultaneously.

His expertise extends to arcade crane games:

Tanjiro recalls Kotetsu explaining how “pity mode” (where the claw sometimes becomes stronger) is a rescue function for terrible players, heavily stressing the “terrible” part and insulting Tanjiro’s skill in the process.

Study Training with Genya and the “Bump Mode”

In one chapter, the main trio (Tanjiro, Zenitsu Agatsuma, and Inosuke Hashibira) visit the Tokito household with Genya Shinazugawa to study math.

Kotetsu arrives because he had promised to hang out with Muichiro that day.

Muichiro suggests using Yoriichi Type Zero to help with Genya’s math practice.

Kotetsu is eager to cooperate and lend the robot.

Kotetsu bluntly points out that Tanjiro’s group has even worse grades than Genya, yet they are not studying.

When they try to excuse themselves by saying Yoriichi Type Zero is already in use for Genya, Kotetsu activates a second unit.

He attempts to start a “bump mode” study session in which the robot literally hits students when they get problems wrong.

However, Yuichiro Tokito returns home, and the others quickly decide to learn from him instead, thwarting Kotetsu’s plan.

Later in the training camp’s second half, Kotetsu still wants to test his system, so he uses Yoriichi Type Zero to capture Zenitsu and Inosuke.

He prepares to force “bump mode” on them, though the story cuts off before showing the actual execution.

From that point onward, Kotetsu appears as a friend of Tanjiro’s group and Genya.

He remains the same: talented, brutally honest, and always ready to weaponize his inventions for “education.”