Well, start by imagining the kind of story you want to tell. Is it a love story that unfolds slowly over time? Or a more action - packed adventure where the reader and Bruce team up to save Gotham? Once you have the basic plot, start writing the scenes. Describe Gotham vividly, from the gloomy rooftops to the seedy underworld. For the relationship, show how Bruce's brooding nature and the reader's unique personality interact. And keep in mind the overall tone of the Gotham universe, which is usually dark and gritty.
First, research Gotham's world thoroughly. Know the different aspects of the city, its dark alleys, and its high - society. For Bruce Wayne's character, study his backstory, his dual identity as Batman, and his values. Then, focus on creating a unique reader - insert character. Give them traits that would either complement or contrast with Bruce's.