Banana Fish is not based on real events. It's a fictional narrative that takes the reader/viewer into a made-up world with its own unique plot and characters.
Banana Fish is purely fictional. The plot, characters, and events were all imagined by the writer to create an engaging and unique narrative. There's no real-life inspiration behind it.
In 'banana fish short story', it focuses on Ash Lynx's journey. He is a character in a rather gritty world. The story may include his relationships with other characters, like Eiji. The plot is centered around the mystery of banana fish, which might be some sort of drug or something more sinister. Ash is constantly in danger as he tries to uncover what banana fish really is and how it affects people.
In 'banana fish fanfic', friendship and loyalty are often explored as themes. Since the relationship between Ash and Eiji is so central in the original, fanfics may expand on that. They might show how their friendship endures through different trials. Another theme can be the exploration of the criminal underworld in more detail. Writers could create new storylines within that seedy world, perhaps introducing new characters who interact with the main cast in interesting ways.