SPOILERS. oh yeah, nezuko was definitely used as a plot device. she was one of the reasons for tanjirou to pursue demon slaying, in order to find a way to bring nezuko back to normal. and then there’s muzan who wants nezuko in order to become immune to the sun. even when tanjirou became a demon and was corrupted by muzan, nezuko was the reason he broke out of it. so yeah, I really do think so!