What are the order of events between Fuka, Desco and Des-X?
Desco was probably built with the image of Fuka in her mind from the get-go; she was intended to be her little sister after all. Plus, even if she wasn't, Genjuro (Fuka's dad) showed Desco photos and videos of Fuka as a little girl, so Desco knew roughly what her Big Sis looked like before meeting her in person. As for the order in which those events happened, it's hard to say, but I assume Des-X killed Fuka then defeated Desco, causing her to end up in Hades; otherwise Desco would have found and met Fuka before ending up in Hades.