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. Later on, Kaigaku went on a solo mission to hunt down Kokushibou, the Upper Moon One, but was overpowered.
When going up against Kaigaku, Zenitsu is stern, he constantly insults Kaigaku and is described as suddenly becoming a different person by Kaigaku himself. Despite how Zenitsu acts towards Kaigaku, in the past he had always respected him even if he hated him.
Kaigaku then uses the Thunder Breathing, Fifth Form: Heat Lightning and taunts at Zenitsu, praising his own Blood Demon enhanced Forms. He explains that once his Thunder Breathing forms slash Zenitsu's body, his skin and flesh burns and cracks apart as he uses the Thunder Breathing, Sixth Form: Rumble and Flash.
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.
He abused Zenitsu Agatsuma. He betrayed the Demon Slayer Corps, driving his master to suicide.
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.
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.
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.
1 Gyomei Himejima - The Stone Hashira
The oldest of the bunch, the Stone Hashira, Gyomei Himejima is 27 years old. He is a giant with a soft heart who cries over insignificant things, but despite all that, he is easily one of the most powerful Hashiras.
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.
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.
As further credence to his strength, Zenitsu was able to advance to the highest point of the Hashira Training. Zenitsu singlehandedly killing an Upper Rank demon.
But the manga shows that he can fight without fainting or going unconscious. Later in the manga, Zenitsu fights an Upper Moon 6 demon named Kaigaku. He was Zenitsu's senior and wielded the Thunder Breathing Style. During that fight, Zenitsu was conscious the whole time.
Upon seeing her for the first time, Zenitsu falls in love with Nezuko at first sight. She apparently views Zenitsu as a "weird dandelion" and did not initially seem to reciprocate his feelings. Although the two are rarely seen interacting together, Zenitsu treats her with affection and care.
Eventually Zenitsu and Nezuko would marry and start a family as evidenced by their descendants.
Shinobu Kocho is a capable Demon Slayer whose worthy of being the Insect Hashira. That being said, she's arguably the weakest Hashira when it comes to pure strength. However, brawn is not the only way to determine power in Demon Slayer.
Despite her petite appearance, Mitsuri Kanroji has a special muscular composition that makes her muscles eight times denser than an ordinary human's. She is also more flexible than the other Hashira. These two strengths balance out her being the slowest runner out of all the Hashira.
But, comparing the two characters in their best forms, Zenitsu might be just a tad bit faster owing to his Godspeed move.
Thunder Clap and Flash—Godspeed is the most powerful of Zenitsu's derivations of the Thunder Breathing first-form technique.
His immense stamina and will, makes him a powerful opponent as he is able to go long periods of time in battle without giving up. Just like his comrade Tanjiro, Zenitsu also refuses to give up in the heat of a battle and was able to battle with extremely stronger demons such as Kaigaku and Muzan.
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.
Away from his fears and anxieties, a sleeping Zenitsu can unleash his mighty thunder breathing skills which allow him to move at lightning speed with his Nichirin Blade. His incredible abilities lay dormant underneath his flighty exterior, and sleeping lets him showcase them.
Despite being a Demon Slayer held in high esteem by way of association to the Demon Slayer Corps, Zenitsu carries himself contrariwise, often rubbing people the wrong way with his speech or his actions. In particular, Zenitsu's interactions with the opposite sex are, in many instances, uncomfortable to watch.