I'm not entirely sure as I haven't read it in detail. But generally, it might be about Guren Ichinose facing some sort of catastrophe when he is 16 years old. It could involve his struggles, growth, and how he deals with the challenges that come with this catastrophic event.
The character of Ichinose Natsumi herself makes these visual novels special. Her unique personality traits, like her kindness and determination, are often central to the story. This makes her a very relatable and engaging character for the players.
Since the title focuses on Guren Ichinose, he is definitely a main character. There might be other characters like his friends or comrades who are also important. Maybe there are characters who are antagonists in the story, who create the catastrophe or oppose Guren during this difficult time. However, without reading the novel, it's hard to name all of them precisely.