Some people like to listen to the audiobook version if it's available. This way, you can 'read' while doing other things like driving or doing chores. It gives you a different kind of experience compared to reading the text.
Their relationship could be developed uniquely by having them exchange roles. For instance, Rias could take on some of Riser's responsibilities in the Underworld for a while, and Riser could do the same in Rias' territory. This would give them a deeper understanding of each other's worlds and the pressures they face. They would then be able to support each other in new ways, leading to a stronger relationship.
Their relationship could develop uniquely if Issei starts to teach Ophis about human emotions more deeply. Since Ophis is a powerful being, she may not fully understand all the nuances of love. Issei could show her through simple acts like sharing his favorite food or taking her to a place that has special meaning to him.
It might be an abbreviation for a name, like 'David X. Doe' or something similar. Or it could be a unique name within a specific group or community that is into fanfiction. Maybe it's a code name for a person who reviews fanfiction. Without more context, it's really hard to say for sure.
The way of expression refers to the language, words, symbols, techniques, etc. used to express the author's thoughts, feelings, and opinions.
Techniques of expression: refers to the various specific methods used to express the author's thoughts, feelings, and opinions, such as metaphor, personification, exaggeration, comparison, parallel, rhetorical question, metonymy, antithesis, repetition, etc.
Rhetoric: refers to the use of various rhetorical techniques to enhance the expression of the article, such as metaphor, personification, exaggeration, contrast, parallel, rhetorical question, metonymy, antithesis, repetition, etc.
Writing techniques: refers to the various techniques and methods used in the writing process, such as description, narration, discussion, lyricism, structure, rhythm, language, style, etc.
A writer could also introduce new plot elements related to this. Maybe there's a curse that needs to be broken, and the story revolves around Issei trying to figure out how to break it. This would create suspense and keep the readers engaged.
I'm not entirely sure specifically what 'dxd adult fanfic' is. 'DxD' might refer to a particular series (such as 'High School DxD'), and 'fanfic' stands for fan fiction. However, the 'adult' part implies it could be fan - created stories with mature or inappropriate content which is not suitable to discuss in detail.