Known for his loud volume and cowardly antics, Demon Slayer's Zenitsu Agatsuma seems like the weak link of his friend trio. He lacks Tanjiro's ambition to keep fighting and doesn't possess the drive as Inosuke. Zenitsu is quick to panic and, if it doesn't involve Nezuko, he will run away rather than fight. His less amiable qualities don't make him seem fit to be a demon slayer, especially compared to some senior members.
Despite his personality, Zenitsu has a strong skill set and unique abilities only he can use. If the Demon Slayer Corp hadn't disbanded at the end of the series, Zenitsu could have potentially become the next Thunder Hashira.
Zenitsu’s Mentor Was a Former Hashira
One of the ways to become a Hashira in Demon Slayer is to be one's tsugoku and train under them. Kanao was the tsugoku to Shinobu, while initially, Senjuro was his brother Kyojuro's tsugoku. Zenitsu trained under the former Thunder Hashira, Jigoro Kuwajima. Under Jigoro's strict training, Zenitsu only mastered the first form of Thunder Breathing. Although he often tried to run away from Jigoro, he highly respected his mentor and viewed him as a parental figure.
A Hashira typically scouts for a successor to take over their position if they die in battle or retire. After Jigoro's death, Zenitsu could have taken over the role of Thunder Hashira. However, he felt extreme guilt over his mentor committing seppuku because one of his students turned into a demon, and instead focused on training to become stronger and avenge his master.
Zenitsu Defeated an Upper Moon by Himself
The second way to become a Hashira is to kill 50 demons or defeat one of the Twelve Kizuki as a kinoe-ranked slayer. Tanjiro and his friends managed to kill several of the Twelve Moons throughout Demon Slayer. Giyu slays Lower Moon Five, Rui, on his own. Tanjiro and co. managed to defeat Lower Moon One, Enmu, with Rengoku's help. Inosuke and Zenitsu beheaded Daki, while Tengen Uzui and Tanjiro defeated Gyutaro.
Only two individuals in the series defeated an upper-moon-ranked demon on their own. Muichiro defeated Upper Moon Five, Gyokko, by himself during the "Swordsmith Village" arc. Zenitsu was the only other person to achieve this feat and defeated the new Upper Moon Six, Kaigaku.
Zenitsu Created an Original Form Within Thunder Breathing
While creating an original form is not a unique trait to the Hashira, it is an extremely difficult feat. Only two known individuals in Demon Slayer have created a new form within a breathing technique that many people already use. Water Hashira Giyu Tomioka created the eleventh form of the Water Breathing technique known as Dead Calm. Zenitsu was the only known non-Hashira to develop an additional breathing form. While training with Jigoro, Zenitsu could only master the first form of Thunder Breathing, while Jigoro's other student, Kaigaku, could use every form but the first. Subsequently, Kaigaku despised Zenitsu and belittled him for his lack of strength.
During his battle with Kaigaku, Zenitsu created the seventh form of Thunder Breathing known as Flaming Thunder God. Kaigaku cursed Jigoro for favoring Zenitsu and teaching him a secret technique but Zenitsu corrected his peer and informed him that the seventh form was his creation -- their master didn't play favorites. This fact both stunned Kaigaku and greatly infuriated him because he always saw Zenitsu to be weaker than him.
Although he comes across as cowardly, Zenitsu's speed, determination and skills give him his own strengths. His training with Jigoro did more than aid him in the art of Thunder Breathing. His master set him up to potentially take over and become the Demon Slayer Corps' next Thunder Hashira.