I'm not entirely sure about the specific plot of 'tomodachi no imouto light novel' as I haven't read it in detail. But generally, light novels often involve various themes such as friendship, relationships, and personal growth. It might be about the relationship between the main character and his friend's sister, perhaps exploring their interactions, the development of feelings, and the challenges they face in their relationship.
Since I haven't read the 'tomodachi no imouto light novel', I can't be certain. But usually, the main character would be the one whose perspective the story is mainly told from, probably a male character if it follows the common light novel trope. And of course, the friend's sister would be a major character. There might also be other friends or family members who play important roles in the story.
The tomodachi game story is quite complex. It begins with a group of friends who seem to have normal relationships at first. But then, they are forced into the 'Tomodachi Game'. In this game, there are rules that often put the friends at odds with each other. The story shows how different characters react differently to the situations. Some might be more self - centered and willing to sacrifice their friends for their own gain, while others hold on to their friendships no matter what. It's a story that makes you think about what true friendship really means and how it can be tested in extreme circumstances.