I need to structure the post with an introduction that grabs attention, sections explaining key points, and a conclusion. Maybe start with an intro about the popularity of video games and the role of fan-made content. Then a section on the origins of doujin games, their importance, and why portability is a key feature. Highlight some examples or case studies if I can find any. Address the legal side to provide a balanced view. End with a summary of how portable doujin games contribute to the gaming world.