Through sheer will and anger, Zenitsu was able to overcome his cowardice and stay awake in battle against demons, shown during his bout with Kaigaku and Muzan, especially in the fight with the latter in which he refused to give up and persisted despite being enervated and injured.
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.
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.
Apparently Zenitsu couldnt use anything else besides the first form of Lightning breath. Later in the manga, it was revealed that he was able to create his own breath technique of Lightning. And he's able to fight full strength without sleeping ..
Zenitsu does, however, appear to retain a certain degree of awareness while asleep. Even during Demon Slayer's "Mugen Train" and "Entertainment District" arcs, he continued fighting in his sleeping state, rather than just unleashing a single blow as he had at the start of the series.
There are six forms, but Zenitsu could only learn the first form said to be the basis for the others yet he honed it to where he could fight against several strong demons. He later created a seventh form and defeated his former senior who had become the Upper Rank Six of the Twelve Kizuki.
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.
When asleep, however, Zenitsu is able to let go of his paralyzing fear and anxiety and let his prowess shine through. In this way, Zenitsu is similar to Rock Lee from the manga and anime series Naruto, since they both need to transport themselves into a different headspace to tap into some of their greater powers.
Goes without saying but, spoiler alert. Yeah he does, the point is that Zenitsu is as strong awake as he's asleep, but he needs to overcome his fears and just show his true potential, which he does.
Zenitsu Agatsuma is the series' sole Thunder-Breather, although Tengen Uzui's Sound-Breathing is derived from Thunder-Breathing.
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.
We hear the Elder Brother Spider Demon say that Zenitsu could barely move his limbs, but he was still defeated. Judging from this, we can say that on a scale of 1–10, Zenitsu's strength is at an impressive 8.5/10.
Zenitsu speed is nearly that of light. To downgrade it, it is still invisible to naked eye, i.e. more than 51 Mach. Zenitsu speed with his sword skills just blitz Kaminari.
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.
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.
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.
As a result of these events, fans have wondered whether Zenitsu has a split personality disorder. The answer to this seems to be no. The reason is actually pretty apparent here. The gold-haired character, even while being cowardly and unwilling, went through rigorous training to master his Breathing technique.
Surprisingly, Zenitsu is the oldest compared to Tanjiro, Nezuko, and Inosuke. What's even more surprising is that even at 16 years old, his cowardly nature makes him seem like the youngest out of the four.
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.
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 is one of the main characters, and he does stop being a coward in Chapter 143–146, which is hte final arc of the series.
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.
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.
1) Inosuke Hashibira
Although Inosuke possesses immense strength and endurance, he's no match for Zenitsu's insane speed and his Thunder Breathing.