The events of this manga is one year after the Kuro no Keiyakusha arc. Therefore, also one year before Ryuusei no Gemini arc. Its like an arc which happened in between. Since Ryuusei is two years after Kuro.
It will not give you spoilers, since the author is obviously clever enough to know not to put anything about Yin's disappearance and why in here. She/he will make it so that Yin disappears after this arc. so most likely we don't get to see it, except in the anime.
So far there has been no spoilers what-so-ever.\, and I'll doubt she'll give any further on in the series.
