I'm not sure exactly which 'ada learns dazai past fanfic' are out there. You could try searching on fanfiction websites like Archive of Our Own. They usually have a wide variety of fanfics on different topics and characters.
Well, first of all, the character of Dazai. His past is often filled with mystery and pain, so in a 'ada learns dazai past fanfic', the details of his past need to be engaging. This could include his relationships with other characters from his past, his motives, and his internal struggles. Then there's Ada. Her perspective and how she processes Dazai's past is significant. Does she empathize? Does she judge? Her actions and thoughts in response to what she learns are key elements. Additionally, the setting of the story, whether it's in a familiar place from the original work or a new location created by the author, can also play a role in enhancing the overall story of Ada learning about Dazai's past.
It's possible that Ada and Claire are part of a fantasy world. Ada could be a shape - shifter or a magical being, and Claire could be a human who has stumbled into Ada's world. The licking of the neck could be a sign of recognition, like Ada is identifying Claire as someone special or as a potential threat in that world.
Ada and Helena are likely characters from a particular fictional universe, perhaps from a game or a book series that has a fan base creating fanfiction about them.
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.