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.
You can start by looking on fanfiction websites like Archive of Our Own (AO3). There are tons of talented writers who create stories about Peter Parker being adorable. Just search for his name and add 'adorable' in the tags. Another option is Fanfiction.net, which also has a large collection of such stories.
One example could be when he tries to cook for his friends in fanfiction. He might be clumsy in the kitchen but his intention to do something nice for them is really sweet. Another is when he gives pep talks to his comrades, his earnestness and the way he believes in them so wholeheartedly is quite adorable.
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.