There's an old anime trope dating back to sailor moon where when women undertake violence they are presented as visually becoming more mature for stepping into the adult world, then regressing back to a childish appearance once the violence is done. Sexist brainworms aside (boy protagonists don't have to put up with that) this kinda works as a visual metaphor when your protagonists are all teenagers straddling adulthood, but this particular use with Nezuko... Nah dawg, the rest of your story about :snipes-hesitation: is fire, but Nezuko's the one glaring weak spot. (and zenitzu, but at least he has moments.)

I really dislike her character design.