Notes:

Sauce: http://www.pixiv.net/member_illust.php?mode=medium&illust_id=48960303

Of note, Alter also has a figure of Kotori in this exact costume coming out in late July and up for preorder here:

http://www.amiami.com/top/detail/detail?gcode=FIGURE-010088&page=top%2Fsearch%2Flist%3Fs_keywords%3Dkotori+figure%24pagemax%3D40%24getcnt%3D0%24pagecnt%3D1