This reminds me to that Kim Possible fanfiction, "Alone Together". I think that was the one that got me into KIGO. Except that Kim and Shego were send to an alternate universe where all people vanished at that same instant.
Back to this one, the ending got me really worried. Multiple bad endings passed thought my head. She really alone and hallucinating, Rii vanishing like the others...
When Rii appeared I felt like dodging a bullet.