One common theme could be the exploration of their relationship dynamics. It might involve how they interact, whether it's in a friendly way or with some underlying tension.
I'm not sure about specific 'once ler x greed ler fanfic' off the top of my head. You could try searching on popular fanfic websites like Archive of Our Own. They usually have a wide variety of fanfictions in different categories.
There are indeed apps for manga reading. For example, MangaZone and Crunchyroll Manga provide access to various manga titles. However, the availability might depend on your region and device compatibility.
Webtoon is pretty good. It offers a diverse collection of manga and often has exclusive content. Also, it's easy to navigate and has nice features like bookmarking and offline reading.
The app itself is free to install. However, depending on the specific manga you want to read, there could be charges for accessing certain volumes or chapters.
Mangatoon is worth checking out. It has a large library of manga, including both popular and lesser-known titles. The app provides a smooth reading experience and has features like bookmarks and recommendations.