Well, some characters might be amused. For example, Atsushi could be a bit shocked at first, seeing the different interpretations of Dazai in the fanfiction. He might find some of the more out - there scenarios quite strange but also interesting in a way.
In many of Dazai's light novels, there are often characters that represent different aspects of the society he was observing. For example, in some of his works, there are male characters who are disillusioned with the social norms and expectations, and their interactions with other characters, such as female characters who might be more traditional or trying to break free in their own ways, create a rich tapestry of human relationships and the exploration of the human psyche within the context of the Japanese society at that time.
Dazai is obviously one of the most important characters. His actions and thoughts drive much of the story. There is also Oda Sakunosuke, who has a significant influence on Dazai's development.
Dazai Osamu himself is of course a very important character. His actions and thoughts drive much of the story. Oda Sakunosuke is another important one. He has a deep friendship with Dazai and his own code of ethics that influences the events in the story.
Yosano could have a rather detached view. She might see the fanfiction as a form of escapism for the fans. However, if the fanfiction involves Dazai in some dangerous or life - threatening situations, she might be a bit more invested, thinking about how she could've saved him in those fictional scenarios.
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.