When Zenitsu was a young boy, he was financially supported by a woman who owned a lot of debt. She had eloped with Zenitsu, which got him into trouble and it was 'Gramps', the former Thunder Hashira, who saved him from the trouble.
Gramps decided to train Zenitsu along with his eldest disciple, Kaigaku. On one of the days during his training, Zenitsu climbed up a tree because he felt that he was disappointing Gramps, when suddenly, a stroke of lightning hit him, causing his hair to turn blonde.
Zenitsu and Kaigaku hated each other; however, Zenitsu respected Kaigaku and treasured him. Zenitsu eventually wished to fight alongside Kaigaku one day, and thus he created a seventh style, 'Breath of Thunder, Flaming Thunder God'.
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 Kasugaigarasu; except Zenitsu who got a sparrow. All four are told to choose what ore they want their sword made out of.
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 cake 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 3 Demon Slayers to a house for them to rest in. An elderly lady attends to them while they are there.
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 style, but honed it to perfection; defeating the demon with it.
Shinobu saves Zenitsu from the poison.