After a long and arduous fight, the Demon Slayer Corps has finally defeated the Sixth Upper Moon demons. Tanjiro and the Sound Hashira, Tengen, were able to behead Gyutaro at great cost to themselves. Zenitsu's incredible speed allowed him and Inosuke to finish off Daki at the same time. The Demon Slayer trio has progressed a significant amount since they were first introduced, but they're still a long way away from reaching the level of a Hashira.
Despite the clear gap in their power, Tengen shows just how strong Zenitsu can eventually become. There are a great number of similarities between the two of them. Both characters possess incredible speed, there's a lot of overlap in their breathing styles and their keen sense of hearing gives them an edge in fights. If Zenitsu could raise these abilities to their highest levels, he could easily reach the power of a Hashira and become unstoppable.
Zenitsu is a Thunder God
All the Hashira have demonstrated insane levels of speed. Tanjiro and Inosuke weren't able to visually keep up with Rengoku's fight with Akaza. When Giyu was first introduced, he was able to take Nezuko out of Tanjiro's hands without him even noticing. Tengen has displayed high levels of speed as well. He was able to deflect a strike that was mere centimeters away from Tranjiro, even while poisoned. A ridiculous amount of speed seems to be a requirement for the Hashira.
Luckily, Zenitsu has plenty of speed. In fact, he is the only non-Hashira Demon Slayer who seems to be able to match their fast movements. With his speed, he was able to completely surprise Daki. His strongest technique, Thunderclap and Flash: God Speed, takes this to the next level. His movements are completely undetectable when he's going at his highest speed.
Only two factors are holding Zenitsu back from reaching his full potential. He's only able to fight at his best while he's asleep, and there's a limit on how many times he can use his God Speed ability. When he's awake, Zenitsu is plagued with self-doubt and worry, which prevents him from fighting at full power. At the moment, he can only use Thunderclap and Flash: God Speed two times, and he severely injures himself while doing it. If he can overcome these limitations, Zenitsu could be as consistently powerful as Tengen.
Thunder and Sound Work Well Together
Thunder Breathing is one of the five main forms of Sun Breathing. It's an incredibly powerful fighting style that utilizes fast strikes and blitzing speed. Zenitsu was only able to master the First Form: Thunderclap and Flash. Despite only knowing one form, he has developed it to such an extent that his entire fighting style revolves around this one move.
Sound Breathing was derived from Thunder Breathing, and Tengen is the only known user of this breathing style. He employs a very unique style of fighting with his nunchaku-like swords and ninja tools. The techniques of this breathing style use explosives to deal damage and fast strikes that take opponents by surprise.
The similarities between Thunder and Sound Breathing mean that Zenitsu could easily add them to his skill set. In the same way that Tanjiro combined Water Breathing and Hinokami Kagura to create something new, Zenitsu could do the same with Thunder and Sound Breathing. If Zenitsu could add his speed to the destructive power of Sound Breathing, he could reach heights untold.
Zenitsu and Tengen Have Perfect Hearing
Besides speed, Zenitsu's most powerful ability is his hearing. His extremely sensitive hearing allows him to fight while completely asleep. He can detect danger from the slightest noise and even tell what kind of person someone is just from the sound of their body. He was able to figure out that Daki was a demon with his hearing alone.
Zenitsu isn't the only Demon Slayer with incredible hearing. Tengen also possesses a superhuman level of hearing. With it, he can mimic echolocation and even identify targets far underground. His most impressive ability is his capacity to analyze the sound of combat and deconstruct it into a rhyme. Once he has finished analyzing the sound, he can read an opponent like sheet music, predicting their attacks and exploiting their blind spots.
Because he possesses a similar level of hearing as Tengen, it would be possible for Zenitsu to learn his Musical Score ability. He has already demonstrated that he has perfect pitch. Even without a musical background, Zenitsu was able to pick up an instrument and master it after just a few minutes. Musical Score is an ability that would not only be easy for Zenitsu to learn but would also fit his fighting style perfectly.
