Chapter 4 of Yoigoshi-hen is out.
Spoiler:
So we now know that Yoigoshi follows the end of Tsumihoroboshi - or rather, the BAD END of Tsumihoroboshi. It doesn't take much then to figure out who "Mion" is - for once, us readers have far greater knowledge and perception of mysteries than characters in the manga itself
How involved was Shion with events in Tsumihoroboshi? Off the top of my head she only appeared when she met up with Kasai and bumped into Rena at the bar. It was Mion that was with Akane when they had that meeting with Ooishi, right?
It's nice to see that Shion has inherited the katana-wielding skills of her mother. Wonder if Kasai will show up with his shotgun?