Zenitsu would rather be anywhere than Mount Natagumo. He is literally shaking in his sandals, and that crushing fear is only compounded when he spots a shack and numerous bodies suspended from threads, the latter in various stages of gruesome transformation. It feels like a nightmare, and Zenitsu just wants to wake from it. Unfortunately, he’s already wide awake, and since he didn’t follow Tanjirou, he’s on his own.
It gets worse: the Spider Demon Brother’s lesser spider minions bit Zenitsu earlier, which means he’s currently doomed to transform into one of the hundreds of grotesque spiders with human heads that are currently chasing him. The Brother doesn’t see Zenitsu as much of a threat, mocking his cowardice. When Zenitsu notices his hair start to fall out, he faints in a tree.
If there’s anything we know about Zenitsu, it’s that he’s at his most effective when he’s unconscious. He recalls the day he was struck by lightning while hiding in a tree, turning his hair yellow. During his ensuing training he was only ever able to pull off the first of six Lightning Breathing Forms.
His master urged him not to be discouraged. If he could only unleash one form, then he needed to perfect that one form, just as a swordsmith hones steel into the hardest, strongest blade. Brother doesn’t expect and isn’t able to deal the fruits of Zenitsu’s rigorous training, and as a result, is beheaded.
While brother’s pocket watch breaks upon his death, his poison is still coursing through Zenitsu’s blood. Zenitsu attempts to slow his breathing and blood flow, the better to hold out if and when someone finds him.
While this episode spent a lot of time fleshing out Zenitsu’s character, it also continues the development of Inosuke as someone who detests being coddled or seen as weak. He laughs off his clearly serious bleeding wounds, and refuses to leave the mountain until the job’s done.
Tanjirou, obviously, isn’t going anywhere until he’s had his shot at the member of the Twelve Kizuki, whose blood could help cure Nezuko. Unfortunately his usually reliable sense of smell continues to be adversely affected by a distractingly acrid odor, such that he doesn’t even notice the presence of the Spider Demon Sister.
Sister gets Inosuke to chase after her, but she’s only posing as a lure for her Father, who looks to be the berserker (and possibly the Kizuki) of the family. Inosuke may end up learning the hard way not to leap headlong into a battle when you’re at less than 100%.