Obviously, Ash and Diantha are the main characters. Ash, with his Pikachu by his side, has been traveling across different regions for years. Diantha, on the other hand, is a highly respected figure in the Pokemon community. In this kind of fanfiction, their personalities and how they interact with each other, especially with the added 'lemon' aspect which could involve romantic or more intense personal interactions, are what drive the story forward.