The mangaer in Dororo is responsible for creating the visual and narrative aspects of the story. They shape the characters and plot through their artistic skills and storytelling abilities.
I'm sorry, I'm not familiar with the specific 'Roy Ruk Raeng Kaen' story. It could be a relatively unknown or regional story. Maybe it's from a particular culture or a local folktale.
A common theme is friendship. Dororo and Hyakkimaru's bond is often explored further in fanfics. Another theme is redemption. Since many of the characters in the original have complex pasts, fanfics may delve into how they seek redemption for their actions.
The Dororo manga has a rather complex and bittersweet ending. Dororo and Hyakkimaru's fates are intertwined in a way that leaves a lasting impression on readers.