Why Zenitsu could not become a Hashira in Demon Slayer. Fans have wondered whether Zenitsu ever got to realize his dream of becoming a Hashira at all. 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.
Overall, Zenitsu is an extraordinary powerful Demon Slayer capable of singlehandedly bringing down an Upper Rank, which said to be able to overpower 5 Hashira, in 10 minutes, albeit he would have beheaded Kaigaku in a heartbeat if it wasn't for his feelings of melancholy and pity.
1) Inosuke Hashibira
Although Inosuke possesses immense strength and endurance, he's no match for Zenitsu's insane speed and his Thunder Breathing.
2) Inosuke
During the Hashira training arc, Inosuke performed exceptionally well. Even Tanjiro needed a few days to pass some parts of the training, while Inosuke seemed to breeze through. His performance against Doma definitely puts him on Hashira's level.
Zenitsu's rankings in the popularity polls are as follows: Zenitsu ranked 2nd place in the first popularity poll with 4,299 votes. Zenitsu ranked 1st place in the second popularity poll with 17,451 votes. Zenitsu also appeared in The Promised Neverland's 2020 popularity poll, ranking 29th place with 44 votes.
10) Zenitsu
He certainly has speed and was able to keep up with Daki during the Entertainment District arc. While some might say that he could one-shot the Upper Moon demon Kaigaku, the demon was nowhere near the level of an Upper Moon since he was not used to his powers.
With all of that, what Zenitsu's true power is his spirit. Zenitsu might be a coward at times, flee from danger instead of facing it head-on, but when he's needed, he'll fight for his friends.
As a result, Tengen Uzui has to deal with the moniker of being the least powerful Hashira around. Of course, that's not the worst thing in the world, as all Hashira are strong.
One plot point fans never did get to see was Tanjiro becoming a Hashira. By the time his promotion would've been necessary, the goal of the Demon Slayer Corps was already achieved. However, this doesn't mean Tanjiro wasn't Hashira level by the series' end, or that he couldn't hang with his colleagues if made Hashira.
However, after the final fight with Muzan the Demon Slayer corps got disbanded. After Muzan was defeated, all the demons he created also perished immediately, so there was no point of a Demon Slayer corps anymore. As a result, Zenitsu did not get to become a Hashira even after becoming strong enough to bie one.
3) Zenitsu Agatsuma
Zenitsu, with his insane strength and lightning speed, can eliminate every Upper-Rank Demon in 1v1 except Upper Moon 1, and Muzan because in terms of strength they are more formidable than him.
Given how talented Tanjiro is, it's likely he'd defeat Zenitsu, even if the latter could fall asleep and call on his powers. Tanjiro's demonstrated his ability to take down more powerful demons and adapt to training on more than one occasion, so it just makes sense.
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.
Zenitsu Agatsuma vs Kaigaku is a battle that took place in the Infinity Castle. It focuses on the battle between Zenitsu Agatsuma against Upper Rank Six Kaigaku.
13/15 Zenitsu Agatsuma Defeated An Upper Moon On His Own
He falls asleep when he crumbles under the weight of his own fear, and his body fights on instinct alone, utilizing his Thunder Breathing Style. He proved that he can be a capable fighter even while awake, defeating Upper Moon 6 on his own.
Intimidation alone would allow Giyu to beat Zenitsu in a fight, but it's also obvious that Giyu's combat skills would allow him to crush the other boy based on physical ability alone. Zenitsu has a long way to go before he reaches Giyu's level.
That being said, Tanjiro's rank is unknown, and his rank was last revealed in the Entertainment District arc. In that arc, they were at Kanoe, which is fourth from the bottom. Since Kiriya Ubuyashiki announced the disbanding of the organization, Tanjiro doesn't get the chance to become a Hashira.
Final Thoughts. Clearly, Tanjiro is much stronger than Zenitsu in Demon Slayer. Tanjiro beats Zenitsu in every relevant category to the question, whether fans directly see this or it's implied. The unique powers of Demon Slayer would normally make it difficult to power scale and compare two characters.
Who was the first and last hashira? Goku isn't a hashira because he is the main character in a "Dragon Ball" and he is a Saiyan. As for the "first ever Hashira"...that is unknown. As for the "last ever Hashira"...that would be Shinazugawa-san as well as Tomioka-san.
The insect Hashira, Shinobu Kocho, is the second active Hashira that fans are introduced to in Demon Slayer, behind only Giyu Tomioka, who makes an appearance in the first episode of the series.
2) Sanemi Shinazugawa
He dedicated all his time and effort towards perfecting his skills as a swordsman which was shown during the fight against both Muzan and Kokushibo.
So in a nutshell, Zenitsu wasn't able to master the other five forms, but later in the Infinity Castle Arc, during the battle with Kaigaku, Upper-Rank Six Demon, he created a Seventh Form: Flaming Thunder God.
Exploring Zenitsu's fighting abilities in Demon Slayer
Zenitsu has perfected the first form only and is seen performing it only when he's unconscious. But the manga shows that he can fight without fainting or going unconscious.
172.1 Km/h Whenever he performs his thunderclap And flash move.