Board Thread:General Discussion/@comment-4317207-20130313085814/@comment-8163713-20130413191734

They're blood related cousins, or rather, they are second cousins. Masaki confirmed in the latest chapter that her and Ryuken are cousins. The first genuine hint to this was during the Fullbring arc when Tsukishima inserted himself as a distant cousin of the Kurosakis who they last met 10 years ago during the funeral of a certain grandfather Rin, incidentally Souken Ishida died around that time. More to the point, Rin can be a nickname.