I'm not entirely sure without reading it, but they might be related to the main characters from 'Rainbow Days'. It could be friends, family members, or even characters who had minor roles in the original but get more spotlight in the side story.
It could be about various things. Maybe it's about the side adventures or experiences of the characters from 'Rainbow Days'. It might focus on their relationships in more depth, or explore some events that are not covered in the main story.
It's mainly about the daily lives and relationships of a group of teenage boys. The story focuses on their friendships, emotions, and experiences growing up.
The Kagerou Days Manga Story has a complex plot involving time loops, different characters' fates, and a mysterious world. It mainly follows the story of a group of teenagers with special abilities. There are conflicts, friendships, and lots of emotional drama.
I'm not sure specifically as I haven't read it. It could be about various things like the adventures of the characters during 340 days, or it might be a side plot that runs parallel to the main manga storyline for that period of time.