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.
One sign is constant criticism. In a toxic friend story, you might notice the friend always finding fault with you, like your appearance or choices. Another is jealousy. If the friend is always envious of your success or new relationships, that's a red flag. Also, if they try to control you, for example, telling you who you can or can't be friends with, it's a sign of toxicity.
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.