The life of a demon slayer is one of hardship. Many who train do not make it through the basics; others die during the final selection. It takes grit, courage, and inner strength to succeed as a demon slayer, all things Zenitsu Agatsuma isn't. Yet, he is one of the most powerful slayers in this world.
As a young boy, Zenitsu had no one until he fell in love. Something that wouldn't surprise fans due to Zenitsu's womanizer tendencies. Wanting to please her, Zenitsu gave the girl a large sum of money, leaving Zenitsu in debt. With the cash in hand, she left him for another man. At first, Zenitsu believed he would have to work off his debt; Jigoro Kuwajima, the former Thunder Hashira, paid off the debt. In return, Zenitsu was to train with Jirgoro to become a demon slayer.
The Boy Struck by Lightning
During his time training, Zenitsu attempted to escape many times, only to be brought back by Jigoro. Zenitsu believed that he would die, either from the training or as a demon slayer. Still, Zenitsu grew in strength, even if he never noticed, and his relationship with his teacher developed as well. Zenitsu took to calling Jigoro 'gramps'. One day while hiding from his teacher, Zenitsu climbed up a tree, where he was struck by lightning. He survived the fatal strike, but his black hair was now blond.
For someone training with the former Thunder Hashira, having survived a lightning strike was undoubtedly a sign of Zenitsu's power. Regardless, Zenitsu still didn't believe he was as powerful as Jigoro claimed him to be, having only grasped the first from, thunderclap and flash. Compared to all the other Demon Slayers who have mastered all the techniques, only having one seems weak, but what Jigoro says makes sense. Zenitsu technically doesn't have to know or master all the forms. If what Zenitsu can do is the first form, then he should perfect that move. Making it impossible for any of his enemies to defeat him, even if it is only with one technic.
For argument's sake, Tanjiro Kamado, a fellow demon slayer who is traveling with Zenitsu, uses the water-breathing form. When battling demons, Tanjiro typically has to use multiple forms to defeat the demon. In contrast, Zenitsu was able to take out the Spider Demon, son, in a single move. He was also able to protect three train carts during the Mugen Train Arc, all while being unconscious. That's the thing about Zenitsu. He doesn't realize how strong he is because his anxiety holds him back. While unconscious, he can have a clear head and fight back, giving Zenitsu an edge over his fellow slayers.
True Power
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. 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. Zenitsu has impressive hearing, similar to Tanjiro's remarkable sense of smell. Zenitsu's hearing allows him to better perceive the events going on around him. He even knew Tanjiro was carrying a demon inside of his box before the truth was revealed. Apparently, all it takes to become a demon slayer is an eccentric teacher, hard work, and a superhuman sense. 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. Having only just met Tanjiro, Zenitsu was willing to take Inosuke's beatings to protect Nezuko's box. Inosuke berated him for not following his duties as a demon slayer and killing the demon inside the box, but Zenitsu refused. He instead kept the box away from Inosuke, protecting it for Tanjiro because Zenitsu knew how important it was to him. Another time Zenitsu displays his fighting spirit is when he is fighting off the demon spider poison. He knows that if he doesn't slow the poison down, he'll transform into a demon and won't be able to help protect Nezuko. Taking the training he learned from Jirgoro, Zenitsu slowed his breathing down, causing his heart to pump slower. This prevented the venom from spreading faster throughout his body.
The world of Demon Slayer is a cold-hearted one, where many are killed or eaten alive by demons. Many who are weak will not survive, but by finding the courage and strength within himself, Zenitsu was able to become stronger than most.