Zenitsu was a supporting character of the Demon Slayer story and an unlikely coward. Despite his gibbering and incompetent mannerisms, he is arguably even more deadly than Tanjiro under the correct circumstances. After he has fainted, he is capable of mastering his "Thunder Breathing" technique and cutting most of the adversaries who might oppose him.
Nonetheless, the aspiring hero is not invincible even in this enhanced state. By identifying anime characters from many different universes that he could defeat and those he'd lose to, we have a better understanding of his true potential.
10 Would Lose To: Kirito On Account Of Durability & Experience (Sword Art Online)
Kirito is a remarkably competent warrior and nearly an undefeated champion of Aincrad (sharing similar success in Alfheim and Gun Gale). He was skilled enough to defeat dozens of seasoned swordsmen at once, possessing such profound agility that he overcomes a vertical obstacle that was supposed to be impossible to beat.
Though Zenitsu is formidable in his unconscious state, much of his threat is on account of his reflexes and intuition, forcing him to rely on overwhelming speed for success. Considering that the Black Sword's haste is comparable ( and with even greater alacrity), he has a palpable advantage over his opponent.
9 Could Beat: Uzu Can Only Mimic Light Speed (Kill La Kill)
Uzu was a member of the Elite Four and became exponentially more difficult to defeat after he upgraded his uniform. Able to transform into a towering warrior, he disoriented foes with attacks seemingly as fast as light speed.
However, his comrades have pointed out that he isn't actually attaining it - what his opponent sees is an illusion that turns their vision against them. Considering that Zenitsu can actually reach the speed of lightning (and since he wouldn't perceive Uzu's feints when unconscious), he would cut him down with little effort.
8 Would Lose To: Killua Zoldyck Can Keep Up With Him & Has An Experience Advantage (Hunter X Hunter)
Killua Zoldyck's "Godspeed" allows him to reach a velocity comparable to Zenitsu's own. He has many advantages that would allow him to dominate his demon-slaying counterpart, particularly through differentials of experience.
The training that the assassin's family put him through enhanced his durability and combat awareness exponentially. Further, it would allow him to realize that Zenitsu only has one move and devise an appropriate way to counter him.
7 Could Beat: Midnight's Quirk Works Against Her (My Hero Academia)
Under ordinary circumstances, Midnight's quirk is a desirable utility when defeating her opponents. A tug of her sleeve allows her to secrete gas that knocks her victim unconscious.
However, this would only accelerate Zenitsu's transition into his enhanced state and allow him to properly enter the fray. Considering that the pro hero doesn't have any weapons to defend herself with (or even especially impressive reflexes, as seen through her defeat by Mineta), it follows that she could not prevent her blond foe from cutting her down.
6 Would Lose To: DIO's Stand Makes Zenitsu's Speed Irrelevant (JoJo's Bizarre Adventure)
DIO's "The World" Stand allows him to freeze time for up to five seconds. This provides an ample window for him to destroy the demon slayer even if he cannot match his pace.
Zenitsu's only reprieve would be if he overwhelmed the vampire before he activated his Stand. However, since his prowess requires him to be unconscious, this is profoundly unlikely. Further, DIO has displayed remarkable durability and regeneration even if he could manage to land a grazing blow (as he survived having his skull crushed).
5 Could Beat: Mine's Weapon Wouldn't Work Against Zenitsu (Akame Ga Kill!)
Mine was Night Raid's sniper and the owner of a curious Imperial Arms. Its strength, range, and caliber all grew proportionately to the danger its user genuinely felt like they were in.
Zenitsu is an already underwhelming individual, though passing out at the beginning of the fight would make his opponent's weapon survivable even if it lands a direct hit. Mine would not have another chance to realize what danger she was in before her head had been cleaved from her shoulders.
4 Would Lose To: Mayuri's Bankai Is Inescapable (Bleach)
Mayuri's Bankai, Konjiki Ashisogi Jizō, was a massive, infant-like demon that secreted a poisonous gas. It covered a wide radius in a stunningly short amount of time, with its owner being the only person immune to it.
This would force Zenitsu to fight aggressively in order to take the captain out before he succumbed to the fumes. However, considering Mayuri's durability (storing several extra vital organs throughout his body and able to survive losing most of his torso in a fight), it follows that he would be able to survive for what little window his opponent has to win the fight.
3 Could Beat: Levi Ackermann Is Only Used To Fighting Opponents Slower Than He Is (Attack On Titan)
Levi Ackermann was the captain of the Scouts and a man responsible for downing its most deadly detractors (such as Zeke and Annie Leonhart). His ODM gear is an excellent compliment to his already sterling utility, occasionally harnessing the mysterious gifts of his lineage in order to perform near-superhuman feats.
However, his speed is relatively abysmal when compared to Zenitsu's, as the latter was capable of beheading a demon in the blink of an eye. Despite Ackermann's superior combat experience, he cannot defend against an opponent too quick for him to perceive, nor does he have the durability to survive him.
2 Would Lose To: Drole Can Affect Large Areas & Has A Durability Advantage (Seven Deadly Sins)
Drole was the Commandment of Patience and the former king of the giants. He possessed enormous tenacity, capable of surviving many of his limbs being chopped off by Meliodas.
His greatest advantage over Zenitsu is on account of the large area he can affect in a limited amount of time. His earth manipulation would be able to seal and crush his diminutive foe from anywhere on the battlefield (as proven through the maze he created against Diane and King).
1 Could Beat: Kurenai's Genjutsu Wouldn't Work On A Sleeping Victim (Naruto)
Kurenai was Konoha's foremost expert on genjutsu, employing it to reliably defeat her opponents. She had all of the standard skills and intuition that her station as a Jonin demand, as seen through her assistance in the battle against Kisame and Itachi outside the Leaf.
However, genjutsu is only able to assail one's mind if the subject in question is actually conscious. Considering that Zenitsu's "Thunder Breathing" only becomes a threat after he has passed out, her greatest weapon is virtually worthless.