Mount Natagumo Arc Zenitsu recalls a moment during in his training when Kaigaku had admonished him for wasting the time of Jigoro, who he had respected, because of his constant whining and cowardice.
Believing he was losing his master's favor, Kaigaku began to despise Zenitsu for being merely a weak and timid man who earned his master's favor out of dumb luck. Thus, he started to bully Zenitsu constantly.
One of the characters that people mistake for Zenitsu's brother in the series is Kaigaku. However, this is not true, and Kaigaku is just his senior in the dojo where the two trained under the guidance of Jigoro, and was also adopted by the former Thunder Hashira.
"Son" of the Spider Family taunts Zenitsu. Zenitsu runs away at the sight of the monster but the demon taunts him by saying that, "You're already losing".
Zenitsu doesn't die in Demon Slayer. In fact, he kills one of the Upper-Rank Demons and works with Tanjiro and the other Demon Slayers in the fight against Muzan.
Unfortunately, Gyutaro spots Inosuke and stabs him right through where his heart should be with a sickle imbued with deadly poison.
Though he eventually avenges his family during the final battle, he loses his left arm and right eye in the process while being fatally wounded. Both are restored during Muzan's attempt to turn him into his successor by transforming him into a demon, though with the aid of his friends, he becomes human once again.
Despite having an intense fear of Demons, Zenitsu develops a crush on Nezuko. He tries to convince her to marry him several times and never fails to mention his admiration for her whenever he sees her. Injured Zenitsu hugs Nezuko.
Through sheer will and anger, Zenitsu was able to overcome his cowardice and stay awake in battle against demons, shown during his bout with Kaigaku and Muzan, especially in the fight with the latter in which he refused to give up and persisted despite being enervated and injured.
The reason for this is pretty simple. Zenitsu wasn't able to master the other 5 forms. He never wanted to learn any breathing techniques. or fighting techniques, nor he wanted to join the Demon Slayer Corps. He was forced, not in a bad way, to train because his master, Jigger Okuwajima, saw potential in him.
Eventually Zenitsu and Nezuko would marry and start a family as evidenced by their descendants.
Nezuko marries Zenitsu Agatsuma in the Demon Slayer series.
He was soon decapitated by the combined force of Kanao and Inosuke, avenging Shinobu and her family. Trying to struggle for survival like Akaza, but lacking indomitable willpower as Akaza had, Doma's head slowly disintegrated and he met Shinobu's soul in the limbo, holding his severed head.
He abused Zenitsu Agatsuma. He betrayed the Demon Slayer Corps, driving his master to suicide.
Because of the movement pattern of the Thunder Breathing's first form, it can be speculated that one reason he had difficulty mastering it was because he placed his scabbard/sheath on his back instead of either of his waists. Kaigaku is the second known Demon Slayer to have become a demon, the first being Kokushibo.
Tanjiro decapitates Akaza with the Hinokami Kagura: Setting Sun Transformation. Akaza was bewildered, wondering how Tanjiro did not have any fighting spirit, comparing this feeling to fighting against a plant. He acknowledges that Tanjiro surpassed his speed.
While unconscious, Zenitsu is a force to be reckoned with, but his actual power is discovered when he is awake. While awake, Zenitsu was able to match the upper rank six, Kaigaku. He was also able to keep up with the Demon King, though he was in a weakened state during this battle.
Try watching this video on www.youtube.com, or enable JavaScript if it is disabled in your browser. The warrior Muzan feared was a swordsman named Yoriichi Tsugikuni. Yoriichi was the only slayer before Tanjiro who came close to killing Muzan, and the flashback of their fight occurred in Chapter 187.
Tanjiro Is Strong, and Stronger than Zenitsu as he uses Sun Breathing.
At the end of the chapter, there is a picture of Inosuke giving acorns to Aoi while she smiles. It is confirmed in the Volume 23 extras that Inosuke and Aoi did eventually end up together and that they have two great-grandsons, one of which is Aoba.
Doma grew fond of her due to her beauty and singing ability, intending to keep her alive until she died of natural causes.
A spider demon drops from the sky and tells Zenitsu that there is no point in running away. The spider demon explains that the spider that bit Zenitsu earlier was one of his, and Zenitsu has been poisoned and will slowly start turning into a spider.
Zenitsu Agatsuma is a part of the Demon Slayer Corps who the previous Thunder Hashira trained. Zenitsu is a skilled and able member of the organization but gets easily scared when facing demons. But as the series continues, we watch him grow as he eventually overcomes his fear of demons and fights them head-on.