The fanfiction 'In Dazai's Shadow' is also worth checking out. In this story, the OC is a shy artist who gradually opens up to Dazai. The story is not only about their relationship but also about the OC's personal growth and how Dazai influences it. The writing style is very descriptive, making it easy to picture the scenes.
One common theme is mentorship. Dazai often acts as a mentor to Atsushi, guiding him in the use of his ability and in navigating the complex world they live in. Another theme is friendship. Despite their differences, they form a strong bond of friendship that is often explored in these fanfictions.
First, you need to have a clear idea of your OC. Decide on their personality, backstory, and what makes them unique. For example, if they're a strong - willed journalist, that will shape their interactions with Dazai. Then, think about the setting, like a modern - day Yokohama or a fictional alternate universe.
Dazai is not based on a true story. Usually, characters like Dazai are formed through the author's imagination and artistic expression to create an engaging narrative for the readers.