Minato Kanae's 'Confession' was a very popular Japanese novel that told the story of a young girl growing up and her complicated relationship with a boy who expressed his true feelings to her. The plot of this novel is compact and emotional. The readers feel a strong emotional resonance in the story and are deeply loved by the readers.
Nezuko is initially 12 years old in the manga. As the story unfolds, her age doesn't change significantly in a traditional sense, but her character development is quite remarkable.
A simple Nezuko and Zenitsu fanfiction could be that they go on a journey together. Along the way, they encounter other demon slayers who are initially skeptical of Nezuko. But Zenitsu stands up for her, explaining that she's different. This shows the depth of their friendship and how Zenitsu has come to trust Nezuko completely.