WARNING: The following contains spoilers for Demon Slayer: Kimetsu no Yaiba Season 2, Episode 10, "What Are You?," now streaming on Funimation, Hulu, and Crunchyroll.
The one thing fans of Demon Slayer: Kimetsu no Yaiba have always agreed on is that Agatsuma Zenitsu needs to chill out. Travel companion and friend of protagonist Kamado Tanjiro, the Thunder-Breathing swordsman has been a (loud) mainstay of the series since the early days. It seems as though Zenitsu was initially meant to be the comic relief, but it's generally only a relief when he stops screaming. Introduced as a Demon Slayer who is terrified of demons, the third episode of the "Entertainment District" arc has shown that Zenitsu has finally grown beyond his crippling fear and panic-stricken scream-rants.
In the past, Zenitsu was so petrified by demons that he would hide behind children, run the other way, or even outright refuse to go on missions. Fear of demons is perfectly understandable, as many of Kibutsuji Muzan's creations are extremely disturbing to look at. Yet Zenitsu's habit of being driven to lunacy by fear was, at times, difficult to endure. There were times when Zenitsu felt truly hopeless as a Demon Slayer, especially considering the demons he encountered in the early days of the series were incredibly weak in the grand scheme of things.
After the death of Kyojuro Rengoku, Zenitsu seems to have had a breakthrough. During the four months that he, Tanjiro and Inosuke spend training after the events of Mugen Train, Zenitsu appears to have dedicated himself to improving his skills as a Demon Slayer. Tanjiro even notices Zenitsu no longer screams and throws fits out of fear of demons, even when he must go on a mission alone. While he's still terrified of demons and confrontation, it's become clear Zenitsu is finally finding his way.
When the "Entertainment District" arc begins, it almost appears as though Zenitsu is reverting to his old ways, given how often he screams at Tengen Uzui. Certainly not a fan of the Sound Hashira, Zenitsu even briefly forgets his mission to find Tengen's wives and decides he will become the greatest Oiran in the city just to spite Tengen. Unfortunately for the Thunder-Breathing swordsman, the instant Zenitsu gathers himself and begins his mission, he finds himself in the crosshairs of the very demon Tengen has brought them to the city to find.
For someone who used to faint at the sight of average demons, Zenitsu impressively maintains his composure when a powerful demon appears behind him without warning. Despite his fear, he stays calm enough to realize the demon behind him must be an Upper-Rank, one of the most powerful in the world. When the demon, later identified as Daki -- Upper-Rank Six of the Twelve Kizuki -- begins to torment a young girl in front of Zenitsu, the hero shows his new spine by angrily grabbing her wrist and demanding she release the girl. Knowing he has no hope of overpowering this demon, this marks the first time Zenitsu does not let his fear control him when it matters.
Although he is quickly overpowered and later captured, Zenitsu has proven that he's truly changed. Given Daki's treatment of Makio, one of Tengen's wives, it's unlikely that she has killed Zenitsu. Despite being a prisoner, this new and improved Demon Slayer appears poised for a breakout performance in the "Entertainment District" arc. Time will tell how Zenitsu can contribute to the inevitable fight against Daki now that he's been caught, but conquering his fear in the face of such a powerful demon is a good sign that Zenitsu may have more surprises in store as Demon Slayer continues.