This entire Doujinshi is based on my personal interpretaion of what Yoruichi and Byakuya might have had before she left Soul Society, starting with the hint that Kubo Tite gave us in chapter 118 :" Did you ever win even once against me in onigoto?" Honestly the first thing that went through my head after I read that line was the picture of a younger (not a child) and more inexperienced Byakuya with her as a sort of teacher/rival/ect. That was before I knew what onigoto was, I had thought it some sort of training/contest. Then I found out that onigoto means "tag", and ever since then I've been dying to find out what she meant by that. Did she really play games with Byakuya when he was younger or was it just training.
121 chapters later ( currently at ch.239) we still don't know and maybe KT isn't even planing on letting us know Anyway , until then , this is what I hope their relantionship was like )