Demeter has no real route, so she's an extra girl that you can do at anytime, in any route. She will be tied to the side quests. Basically a reward of doing side quests, alongside the possibility of updating your status +1 times a day.How do you get that Riveria route where she trains you? Also, is Demeter needed for the harem route? I somehow missed her on my harem route save. I put that save on hold and doing Airmed right now so I don't know which all girls are in the harem.
I say "will" because someone once reported her boobs are too small in some scenes, so I decided to simply rework her whole "route" at some point.
At this point, I think Demeter will be one of the last things I will be doing, alongside the Freya/Ishtar achievement scenes and other side stuff.