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.
Zenitsu defeats Kaigaku with Seventh Form: Honoikazuchi no Kami.
He had a younger brother (Zenitsu) and two younger sisters (Ayeka and Kina)—all of them succumbed to despair once they learned of his death. After he learned the news of his younger sister`s death, Kaigaku was devastated and sent a gift back home for his younger sister - a pressed flower crown - that went on her grave.
He abused Zenitsu Agatsuma. He betrayed the Demon Slayer Corps, driving his master to suicide.
When Zenitsu sees her for the first time and discovers that she is a cute girl, he quickly gets mad at Tanjiro because he thinks that Nezuko is actually his girlfriend instead of his sister.
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.
After his treachery, Kaigaku later becomes Zenitsu's archenemy. His cowardice would later lead him to his downfall when Zenitsu overcomes his insecurities as Kaigaku mocks Jigoro. Kaigaku's Thunder Breathing training would lead to his Blood Demon Arts being related to lightning.
Eventually Zenitsu and Nezuko would marry and start a family as evidenced by their descendants.
Nezuko marries Zenitsu Agatsuma in the Demon Slayer series.
This mastery allowed Zenitsu to form a Seventh Form of Thunder Breathing that is unique only to him: Honoikazuchi no Kami. While this move might seem similar to Thunder Breathing's First Form, the speed and power of this technique are on a whole new level.
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.
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 is a demon affiliated with the Twelve Kizuki, holding the position of Upper Rank Six ( 上 じょう 弦 げん の 陸 ろく , Jōgen no Roku?), replacing Daki and Gyutaro after their deaths.
Unfortunately, the answer to this question would be, no, he did not. Zenitsu, along with Tanjiro and Inosuke got way stronger than average corps members with training. In fact, he became one of the very few to even invent a new form of Thunder Breathing. However, his dream of becoming a Hashira was never realized.
Unfortunately, Gyutaro spots Inosuke and stabs him right through where his heart should be with a sickle imbued with deadly poison.
Doma grew fond of her due to her beauty and singing ability, intending to keep her alive until she died of natural causes.
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.
After the battle, Tanjiro returns home with his sister and friends. Later it is revealed that he married fellow slayer Kanao Tsuyuri.
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.
When facing Kaigaku during the Infinity Castle arc, Zenitsu debuts his brand new exclusive Seventh Form to Thunder Breathing. With this, Zenitsu finally has more than one tool in his arsenal and has overcome his own personal doubts and fears. While Zenitsu's character arc is short, this is a fantastic ending to it.
He killed himself because his other pupil, Kaigaku, became a demon. To show remorse for his pupil becoming a demon despite being a demon slayer, he killed himself.
Zenitsu's debt is estimated to be around 600,000 yen in modern currency. This massive amount of debt likely came from his 7 ex-girlfriends.
His love interest that develops as the story goes on is Kanao Tsuyuri and their romance is quite adorable.