My friend and I have theorized that the ghost girl is Aster.
In another room in lumoise toward the subway theres a poster that says "im going to go for help, meet me in the usual place". We think that whoever the letter was addressed to didnt show up and the girl died from whatever/whoever was ailing her. Thus why she says that you are not the one. But who didnt show and why?
Why does the ghost girl appear in oras at mt. Pyre as well NOT as a ghost? Remember oras takes place before kalos. Zinnia was supposed to meet Aster in Kalos but had to make sure team magma/aqua were properly coerced to bring the primals back (and by extension ensure rayquazas return). Aster and Zinnia were probably frustrated that the meteor was approaching according to the legend and they needed more help, so she left a note in lumoise for zinnia to find after she got off the train. But since Zinnia never came, and she died waiting for her by unknown causes.
Perhaps zinnia was supposed to deliver whatever ninja said will trigger the if else statment. In any case, its not implausible to think zinnia wasnt the only active draconid, nor that she had to sacrifice her meeting with aster to fulfill her mission. Discussion encouraged.