It all depends on who he is and what he wants in general (like why would he know kasumi?), mabye well get lucky and chapter 74 will have him having a memmory or flashback that makes his identity obvious (already said my opinion on that wont repeat it).
Dont think the last idea would work though, Eve and Kasumi may look very similar but their personalities are very different. And saten's "how about this ive fallen in love with you" line seemed like a lame excuse, plus he seems to have more complex agendas(that involve most of Blades group) than such a simple reason.
But then again stuff like that has happened before in this manga.
http://mangahelpers.com/downloads/read-online/45011/16
and past enemies getting much more intimate with each other
http://mangahelpers.com/downloads/re...5?t=1258915941
http://mangahelpers.com/downloads/re...8?t=1258949272