Zenitsu Agatsuma is one of the main protagonists of Demon Slayer: Kimetsu no Yaiba and along with Inosuke Hashibira, a travelling companion of Tanjiro Kamado and Nezuko Kamado. He is also a Demon Slayer in the Demon Slayer Corps.
Master Swordsman: Zenitsu is an incredibly talented swordsman, especially in his proficiency in extremely fast Iaijutsu-like swordsmanship and the Thunderclap and Flash technique, to the point he can create offshoots of the technique and utilize it even while he is asleep. His swordsmanship skill allowed him to fend off dozens of spiders and defend 3 train carts that Enmu was constantly attacking. Zenitsu's sword wielding skill evolved to the point that he could fight against Upper Rank demons of the Twelve Kizuki and later matched a severely weakened Muzan, implying that his swordsmanship skill has most likely improved to the point that it matched that of the Hashira, the highest-ranked and most powerful swordsmen in the Demon Slayer Corps.
Thunder Breathing: A Breathing Style Zenitsu learned from a cultivator and his teacher, Jigoro Kuwajima, but failed to master all the forms in the swordsmanship style but could only be able to use the first of the six forms in it, known as Thunderclap and Flash. However, Zenitsu has become incredibly proficient in fighting and winning using the first form alone, even creating several variations and modifications to it and being able to successfully perform the technique even while unconscious. Later, Zenitsu even created an entire new form in Thunder Breathing, something only a Hashira, specifically Giyu and Muichiro, has done before. This form allowed him to turn the tables against his opponent, Kaigaku, and overpower and decapitate him instantly.