Not overrated at all, he's kind of under appreciated (but I guess that's because lots of people in this fandom haven't read the manga) Even without his mark, he killed an upper moon in one hit. He's mastered the first form to utter perfection making it deadly and he even made a godspeed version.
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.
Regardless Zenitsu's power rivals those of the Hashira. Having the ability to fight unconscious and fight while awake to face one of the upper ranks is a sight to behold. There is more to Zenitsu than is insane strength and power.
Enhanced Strength: Zenitsu has attained amazing physical strength through years of harsh training. Zenitsu can hold down Inosuke Hashibira with his bare hands and throw his entire body by grabbing to and swinging one of his legs.
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.
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.
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.
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.
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.
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.
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.
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.
After mastering his Thunder Breathing, Zenitsu would eventually become capable of awakening his Demon Slayer Mark. As this mark would enhance his strength and speed, he can overwhelm Gyomei to a certain extent, but he still won't manage to defeat him.
9/10 Zenitsu Agatsuma Swiftly Defeated Kaigaku With A New Technique. Kaigaku was originally a disciple of Jigoro Kuwajima along with Zenitsu.
Zenitsu's weakness is also a strength
In fact, he tends to repel women and earn much-deserved slaps in the face when he crosses the line. As a side effect of his cowardice, Zenitsu really wants to marry a girl before he inevitably dies in combat.
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 is an incredibly powerful Demon Slayer, but he's always in fear. His nerves calm down when he's asleep, so that's when he awakens his “powers”. MANGA SPOILER AHEAD!!! Zenitsu becomes a lot more courageous and stands up to a powerful demon while wide awake.
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.
Zenitsu is at least close to Hashira level by the end and Godspeed has limited uses because it disables his legs after those uses. Zenitsu's faster but if this was an actual race or marathon, Shinobu would beat him.
Kaigaku overpowering Zenitsu with his Blood Demon Art-enhanced techniques. Overall Abilities: As a member of the Upper Ranks, Kaigaku is immensely powerful. Combined with his swordsmanship skills as a proficient Demon Slayer, Kaigaku is an extremely deadly demon.
Nezuko Kamado
Zenitsu also protects Nezuko multiple times when she is in danger, evident during Zenitsu's first meeting with Inosuke Hashibira and the battle against Enmu in the train. Eventually Zenitsu and Nezuko would marry and start a family as evidenced by their descendants.
1/7 Seventh Form: Honoikazuchi No Kami
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.
Zenitsu creates his own form called Seventh Form: Honoikazuchi Nokami. Through this form, he is able to conjure a creature akin to a dragon that breathes thunder and fire. When combined with God Speed, his power and strength increase, giving him the ability to defeat demons like Kaiju, from the Upper Moon Six.
Thunder Breathing has 6 standard forms in total, with the first form said to be the foundation for all the other forms. However, Zenitsu Agatsuma developed a Seventh Form in order to stand on equal ground with Kaigaku, his former senior disciple under Jigoro who had become the Upper Rank Six of the Twelve Kizuki.