I'm not sure about its popularity. It might be popular within certain niche communities who are fans of visual novels, especially those interested in stories with female - centric characters. However, without more data on its sales or community following, it's hard to say for certain.
I'm not entirely sure specifically as I haven't fully explored it myself, but generally a visual novel is a form of interactive fiction. It might involve stories about relationships among characters. Maybe it's about different aspects of the relationships between 'kanojo' (which might mean 'she' or 'girlfriend' in Japanese).
I'm not sure specifically what the'sama sama letter day story' is about as it's not a widely known common story. It could be a very personal or a story from a specific local or cultural context.
Maybe the Shinigami uses its special powers to heal Naruto's severe injuries. For example, it could transfer some of its own spiritual energy to Naruto to revive his fading life force.
I'm not sure about its popularity. It could be popular among fans of visual novels, especially those who enjoy stories focused on relationships. However, if it hasn't been widely promoted or if it has a niche target audience, it might not be as well - known in the broader gaming community.
Kimimaro could be an interesting missing nin character to explore in fanfiction. He was a tool for Orochimaru, but had his own internal struggles. You could write about his life before fully becoming a missing nin, his loyalty to Orochimaru despite his own deteriorating health, and what might have happened if he had broken free from Orochimaru's control. Another character could be Nagato. Before he became Pain, he had experiences that could be considered as leading towards a missing - nin - like existence. His views on peace and his use of the Akatsuki could be delved into in the context of a missing nin story.
Her novels often feature strong female characters. For example, in 'Delta of Venus', the female protagonist's exploration of her own sexuality is central. These characters are complex and not afraid to explore their desires.