I vote for a spin-off about her brother Keita.
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I really liked how he would defend his sister even when he was much younger than her bullying classmate. Or how he would test her boyfriend but then recognized how Kotaro's influence was a positive one for her. Not to mention when he apologized to his new in-laws because he wrestled with the notion of calling them "family". In a nutshell, he had his convictions but he was fair and would rethink things. Furthermore, he'd take a step further and take action about it. Yep, yep, yep! I vote for a story about him and how a romance blossoms for him.(▰˘◡˘▰)
FYI, Chapter 2 was repeated so chapter 3 is missing. Thank you.