Zenitsu Agatsuma/Synopsis

History
Zenitsu was not raised by his parents so he did not have any proper guidance. When he was young, Zenitsu fell in love with a girl and decided to give her money to appeal to her, however the girl ended up running away with another guy leaving Zenitsu drowning in debt. Seeing that he was in financial trouble with debt collectors, Jigoro Kuwajima paid off the debt for Zenitsu and decided to train him as a swordsman.

Zenitsu began to refer to Jigoro as 'Gramps' and began to train alongside Gramps' other disciple, Kaigaku. One day when he was training, Zenitsu climbed up a tree out of fear that he was being trained to death and as he began to panic, a stroke of lightning hit him causing his hair to turn blonde.

Zenitsu and Kaigaku hated each other. However, Zenitsu still respected him as his senior and for his strong ambition. Zenitsu wished to fight alongside Kaigaku as equals one day and developed his own seventh form, Honoikazuchi no Kami.

Final Selection Arc
Zenitsu is first seen at the beginning of the Final Selection at Mt. Fujikasane.

After the Final Selection, Zenitsu is among the four people who have passed. Zenitsu is seen talking to himself; fearing he will die one day. Each one received a Kasugai Crow; except Zenitsu who got a sparrow. All four are told to choose what ore they want their sword made out of.

Tsuzumi Mansion Arc
Zenitsu is seen begging a random girl to marry him. Tanjiro Kamado stops Zenitsu, and excuses the girl. Zenitsu explains that he must get married before his next mission, because he might die; he asks Tanjiro to protect him until he gets married. Tanjiro asks him why he became a Demon Slayer. Zenitsu got swindled by a woman, and racked up debt. An old man paid off his debt, and trained him. Later on, the two share a rice ball as they travel to the south for their next mission. Zenitsu and Tanjiro arrive at a large house in the woods. Tanjiro smells an unfamiliar scent, and Zenitsu hears a strange sound. There are two kids nearby, frozen with fear: Shoichi and Teruko. Tanjiro shows them Zenitsu's sparrow; to try and calm them. The kids tell them about a monster who has taken their brother inside that house, and that they tracked the monster here. Tanjiro ensures them that him and Zenitsu will rescue their brother, and defeat the monster.

Zenitsu asks about a non stop sound of a tsuzumi strike. Once the rest finally hear it; a bloody body is thrown out of the house, and lands in front of them. The body is not that of the children's brother. Tanjiro leaves his box with the two kids for protection. Although Zenitsu refuses to enter the house at first; he forces himself to join Tanjiro.

Once inside; Tanjiro tells Zenitsu about his broken bones that haven't healed fully yet. Which scares Zenitsu even more, since he can't rely much on Tanjiro for protection. The two kids are frightened by scratching coming from the box, and come inside the house after Zenitsu and Tanjiro. The house begins to rumble, and Tanjiro and Teruko are teleported to a random room in the house; while Zenitsu is left with Shoichi. While searching for the exit, Zenitsu and Shoichi find Inosuke; who runs past them. A scared Zenitsu and Shoichi wander through the house when a demon sneaks up on them. A demon corners the two in a room, and Zenitsu passes out from fear. As the demon tries to strike them; Zenitsu cuts his tongue off. While unconscious; Zenitsu quickly defeats the demon. Zenitsu wakes up with no recollection of his actions, and thanks Shoichi for defeating the demon.

Zenitsu and Shoichi enter a room, and the room rotates; throwing them out of a window. Once outside, they see Inosuke run out of the house. Inosuke attempts to attack Nezuko who is inside the box, Zenitsu protects Nezuko's box from Inosuke. Zenitsu explains his great hearing, and that he can hear living things breathing, heartbeat, and their blood circulation. Zenitsu knew Tanjiro was traveling with a demon, and tells Inosuke to back off; as he is going to ask Tanjiro himself; why he's traveling with a demon. Inosuke continuously attacks Zenitsu; in attempt to kill Nezuko. Tanjiro attacks Inosuke to stop him, and the two begin to fight. Inosuke passes out from Tanjiro's headbutt.

After burying the dead they found inside the house; a Crow sends the three Demon Slayers to a house for them to rest in. An elderly lady attends to them while they are there.

Natagumo Mountain Arc
Zenitsu, Tanjiro and Inosuke head to Natagumo Mountain for their next mission. They reach their destination at night. They meet a Demon Slayer laying on the ground; who begs them for help as he is pulled into the forest by spider threads. Out of fear Zenitsu refuses to go into the forest; Tanjiro and Inosuke proceed without him. Zenitsu contemplates about not joining his friends. Zenitsu realizes that Tanjiro took Nezuko along with him, and runs in after them to protect Nezuko. While running through the forest; Zenitsu sees a human head with a spider's body. Zenitsu runs away only to stumble across the demon responsible for turning these humans into spider slaves. Zenitsu is bit by a spider, and only has 30 minutes before he also turns into a spider. Zenitsu faints from fear, and begins to fight while unconscious. Zenitsu has a flashback of his teacher who encouraged and never abandoned him, and his senior disciple who always looked down on him. Zenitsu is only able to do the first form, but honed it to perfection; defeating the demon with the Thunder Breathing, First Form: Thunderclap and Flash, Six Fold.

Shinobu saves Zenitsu from the poison.

Rehabilitation Training Arc
He complains to Kiyo about the medicine he has to take. Tanjiro, Goto and Aoi Kanzaki arrive, with the latter scolding Zenitsu for his constant complaining. Tanjiro and Zenitsu renuite, the scarred youth asking his friend if he is alright, and Zenitsu responding by bitterly complaining about everything he has been through since the Natagumo Mountain incident. Tanjiro asks Zenitsu about Inosuke and Murata, and the towheaded youth tells him that Inosuke is right next to him. Tanjiro asks Inosuke to forgive him for not coming back to help him, but is told not to worry about it. Dumbfounded by Inosuke's different sounding voice, Tanjiro is told by Zenitsu about their friend's condition: a badly injured throat.

After a rundown of Tanjiro, Zenitsu, Inosuke, and Nezuko's respective conditions, the four comrades take a much deserved rest. They are later visited by Murata, who explains what happened to him when he was summoned to the Hashira Meeting, and Shinobu, who tells them that their Rehabilitation training was about to begin.

Zenitsu muses over his medical treatment and the condition of the other people turned into spiders. He then thinks about Shinobu and her treatment of these individuals, noting that her sound is irregular and somewhat unsettling. He goes on to note that Tanjiro and Inosuke had been taken by her to undergo the Rehabilitation training and had come back extremely exhausted and bedraggled, causing him to despair, as he was to join the training the next day. The next day, Zenitsu joins his friends at the training, where Aoi Kanzaki explains that there are three parts: a massage to loosen the body, reflex training with Kanao involving medicated bath inside teacups, and whole body training, which is essentially tag, with Kanao and herself.

An irate Zenitsu takes his friends aside and chastises them, annoyed that they had been "fooling around" with girls but had come back to the room looking so miserable. A confused Tanjiro asks his towheaded friend what in the world he is talking about, but the maddened youth tells him to shut up, explaining that this training involves a lot of physical contact, which essentially translates into being touched by the girls on a frequent basis, something he sees as pure bliss. Inosuke, also irritated at this point, says that Zenitsu doesn't know what he is talking about, stating that he'll die inside when he loses to someone with a smaller body, only to mocked by his friend. All three of them then got started with the training, with Zenitsu succeeding due to his love of women, but Inosuke making things difficult for everybody due to his hatred of losing.

Despite the overall positive performances of his two friends, Tanjiro continues to lose. He is soon joined by his two fellows, however, as neither of them can best Kanao at the reflex training or the whole body training. Over the next five days, the three continue their efforts to beat her, but none of them succeed, causing Zenitsu and Inosuke to give up. Tanjiro continues to persevere, but is unable to make any progress, continuing to lose for another ten days.

Having just witnessed Tanjiro beat Kanao during his rehabilitation test, Zenitsu and Inosuke stagger as they try to learn Total Concentration Breathing from Tanjiro. However, despite numerous attempts, the two boys were unable to grasp the concept of the technique and struggle to master it. During one of Tanjiro's lessons, Shinobu interrupts to provide guidance, and words of encouragement to both Inosuke and Zenitsu. The methods she uses to encourage them include, doubting Inosukes ability in order for him to get fired up, as well as tell Zenitsu she's personally "cheering" for him specifically so he gets excited.

Nine days pass after her visit, and both Zenitsu and Inosuke have successfully mastered the breathing technique. Elsewhere, Zenitsu is seen leaning up against Nezuko's box as he explains to her the events that had occured that day, while telling her that he'll take her out at night to see flowers that bloom in the garden.

After having said goodbye to Sumi, Naho and Kiyo, the trio arrive at the train station. Zenitsu begins to act erratically after realizing that they weren't given any orders and could've remained at the Butterfly Mansion. While trying to deflect Zenitsu's assault, Tanjiro explains his reasoning that they needed to leave in order to find Kyojuro and that it would be easier to receive orders while they're in public.

Mugen Train Arc
Tanjiro, Inosuke and Zenitsu stand in front of a train. Inosuke, who has no idea what a train is, describes it as the master of the land and plans to attack it. Zenitsu tries to explain it as a steam engine, but Tanjiro interjects by suggesting that it is a guardian spirit and that they should not attack it. Zenitsu finally explains that it is a vehicle that carries people.

Unsurprisingly, Inosuke charges headfirst into the side of the train, which causes a run-in with the police due to the fact that they have swords and also the fact that Inosuke headbutted the train. Soon after, Zenitsu explains that the Demon Slayer Corps are not officially recognized government and that they should conceal the fact that they carry swords. Tanjiro also notes the train is called the 'Infinite Train' and that Kyojuro has probably boarded it already, as Zenitsu proceeds to buy tickets for the train. As they enter the train, Inosuke is his own boisterous self, screaming about being inside the train and causing a ruckus. Zenitsu asks about Kyojuro and Tanjiro describes him as someone with flashy hair and notes that he is very close. The trio suddenly hear loud clamors of "Tasty!" inside the train and trace it to Kyojuro. He is described as a glutton by Zenitsu upon meeting.

As a demon appears from the opposite end of the train, Kyojuro notes that he was unable to sense him, but proceeds to draw his sword and manages to finish the demon. Much to their awe, the trio acclaim Kyojuro to train them to which he agrees. Thus, attributing them to call him 'Aniki'. Subsequently, Kyojuro and the trio fall asleep on the train.

Within the train, Tanjiro, Kyojuro, Zenitsu, and Inosuke can been seen sleeping soundly. Zenitsu dreams of having a romantic date with Nezuko in a peach orchard.

It is pitch black inside Zenitsu's subconscious, as an Enmu's minion stumbles around trying to find his spiritual core. Abruptly, the sound of sharp blades breaks the silence and Zenitsu appears behind the minion holding a large pair of shears. Enraged at finding a boy inside his subconscious, he says only Nezuko is allowed there and attacks the minion.

Later on, a sleeping Zenitsu Agatsuma drops in front of Nezuko and manages to not only set Nezuko free but also the other passengers around them in a flash by using Thunder Breathing, First Form: Thunderclap and Flash, Six Fold, he declares that he will protect Nezuko and she looks on in awe. Her admiration is quickly dispelled however, as he pops back asleep mid sentence and begins to snore.

Both of them defend three cars of the train from Enmu. After the train is stopped and during the fight between Kyojuro and Akaza, Nezuko falls asleep on the arms of a sleeping Zenitsu as well. After the sunrise, Zenitsu gets her into the box to protect her from the sunlight.

Some time later, Zenitsu informed Tanjiro and Inosuke of Kyojuro's heroics, in which he had used an extensive amount of his techniques to save the passengers when the train derailed. He then asked Tanjiro if an Upper Rank had really come and if they were that strong, which the youth confirmed. A tearful Tanjiro expressed his lack of strength and aid to Kyojuro's fight, wondering if he could ever be like him.

Inosuke suddenly burst into the conversation, berating his comrade for his timid words, calling them pointless, and stating that they simply had to live up to their superior's expectations. He informed his comrades that all creatures eventually die, and that they shouldn't shed a tear out of regret, regardless of the cause of it. Zenitsu points out that Inosuke is also crying, causing him to be headbutted by the latter, who also starts beating on Tanjiro, telling him they need to start training as a pair of Kakushi run up to the three.

At the Butterfly Mansion, Zenitsu mulls over Tanjiro and Inosuke's sadness due to Kyojuro Rengoku's death, bringing the former some buns to cheer him up. A distraught Kiyo accidentally bashes her head into Zenitsu's, crying out that Tanjiro is missing. An apologetic Kiyo further explains to the disoriented Zenitsu that Shinobu had gotten angry at Tanjiro for training while still injured and he had been instructed to rest. Remembering the extent of his friend's injuries, Zenitsu questions his intelligence.

Entertainment District Arc
Four months after the passing of Kyojuro Rengoku, the three boys have been training relentlessly at the Butterfly Mansion while occasionally leaving on individual missions. As Zenitsu goes on solo missions, he becomes less of a whiner.

Since Tengen Uzui is unwilling to give Aoi Kanzaki up for his mission, Tanjiro demands that he gives her up and offers to go in her place. At that moment, Zenitsu and Inosuke arrive to help protect the girls, and they also offer to go on the mission as well.

Swordsmith Village Arc
Zenitsu, after the events of the Entertainment district Arc, woke up two days before Tanjiro did and went on a mission reluctantly.

Legacy
Following the battle against Muzan, Zenitsu would live alongside Tanjiro, Nezuko and Inosuke. At some point he would marry with Nezuko and start a family that would eventually yield great-grandchildren in the present day, Yoshiteru Agatsuma and Toko Agatsuma. He would write the adventures of the Demon Slayer Corps and their final struggle against Muzan in a book.

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Зеницу Агацума