I didn't like them that much at first, but Eirin x Kaguya is growing on me. Or maybe just Eirin --> Kaguya.
Eirin always had this kind of dark air, so I feel that the mood of this doujin was perfect for her, though perhaps unexpectedly uplifting - that Kaguya found humanity in her inhumanity seems beautifully ironic.

I loved how he added so much depth to Eirin and his motivations.
Great story.