On the last chapter a reader reviewed and brought up some good points about the chapter and reminded me to add a little extra to help everyone understand the first chapter before moving on with the story.I want to thank the reviewer for bringing up some good questions and points.
An explanation for Hikaru being adopted by Ai and Hakase is because something happened to Hikaru's father that brought her twin down to visit her. Her mother very much disliked Hikaru and would have done something to her if her father wasn't around to witness it. She lied to her twin about them about being twins and made him believe it was a lie of sorts and had her twin stop coming all together.
As for Ai and Hakase adopting her after she wakes up is that when she got shot when she was really young, Aine saw the aftermath of the shooting on the news and wanted Hakase to find her and look after her when she got out, well awoke from her coma. Shortly before Aine tried to commit suicide and ended up in a coma because of it, he asked Hakase to find a way to look after Hikaru if he ever found her. Aine had talked to Reiji about her and asked him to be friends with her if he ever met her.
The first chapter takes place half a year before the first season of the show and then skips a year to where the second season starts. Hope that helps with the timeline. Not completely sure when Ai was created, or when Aine ended up in the coma. If you have a vague idea please let me know via a review.
Hopefully this little chapter helps with any confusion anyone had about the first chapter, or any later chapters I put up. Thank you again to the reviewer that had good questions about the first chapter and I hope this answers some of the questions you had.
If there are any other questions about Hikaru and Nagi will ever see each other or if they will ever be reunited as a family that will be answered in upcoming chapters.