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 Agatsuma is a part of the Demon Slayer Corps who the previous Thunder Hashira trained. Zenitsu is a skilled and able member of the organization but gets easily scared when facing demons. But as the series continues, we watch him grow as he eventually overcomes his fear of demons and fights them head-on.
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.
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.
The more he came across powerful enemies, the more he realized his need to become strong and protect the ones he cared about. Zenitsu's desire to protect Nezuko especially gave him courage and made him realize the strength he'd had all along.
At this point Zenitsu has manifested his speed and power, and he was also able to keep up with the Demon King, Muzan, even when weakened. It's possible to say that his power easily matched the strength of a Hashira.
Tanjiro Is Strong, and Stronger than Zenitsu as he uses Sun Breathing.
He was forced(not in a bad way) to train because his Master Jigoro Kuwajima saw potential in him, but despite of rigorous training He could not learn more than one form. He has only learned the first Form and has mastered it. Mastered it in a sense that he can use technique multiple times (six fold).
Thunder Clap and Flash—Godspeed is the most powerful of Zenitsu's derivations of the Thunder Breathing first-form technique.
Kaigaku overpowering Zenitsu with his Blood Demon Art-enhanced techniques.
2) Tanjiro Kamado
With Sun Breathing, which is regarded as the paramount of all Breathing Styles, Zenitsu will have a hard time subduing Tanjiro and will be defeated by the latter in no time.
Zenitsu is an incredibly powerful Demon Slayer, but he's always in fear. His nerves calm down when he's asleep, so that's when he awakens his “powers”. MANGA SPOILER AHEAD!!!
Tanjiro's confidence alone would allow him to overpower the other boy, but his knack for strategizing and skill with a sword would certainly help too. Given how talented Tanjiro is, it's likely he'd defeat Zenitsu, even if the latter could fall asleep and call on his powers.
Zenitsu — Anxiety
To the surprise of absolutely nobody, Zenitsu is anxiety incarnate. He starts out the series so paralyzed by fear that he can't even fight when he's conscious, and so he's only able to fight when he's passed out.
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. While Zenitsu's character arc is short, this is a fantastic ending to it.
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.
But, comparing the two characters in their best forms, Zenitsu might be just a tad bit faster owing to his Godspeed move.
1) Tanjiro Kamado
Although he and Zenitsu along with Inosuke, started training together, he excelled both in the matters of strength and swordsmanship. Tanjiro with his Sun Breathing is more powerful than Zenitsu.
3) Zenitsu Agatsuma
If he could, Rengoku would win simply because Zenitsu cannot perform Godspeed more than twice. Since Zenitsu only knows the first form, Rengoku would know what to expect and accordingly counter his moves, taking him down in the end.
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.
During their fight, Zenitsu was at a major disadvantage against Kaigaku until he used the seventh form called “Honoikazuchi no Kami,” literally meaning “Flaming Thunder God.”
slow ?! , Deku was able to dodge and hit in a split second. And yet he is clearly faster than Zenitsu .
While awake, his nerves and fear cause him to freeze up so he cannot move. When his life is in danger, his nerves and fear grow so strong that he passes out. But while asleep, his nerves are no longer holding him back, allowing him to fight and move with great judgement, precision and skill.
1. Gyomei Himejima. Gyomei Himejima, the Stone Hashira, is the strongest Hashira in the Demon Slayer corps in terms of strength and swordsmanship. He is the only person who uses spiked flails and an axe instead of standard swords to kill demons.