So this is a prequel to And Then, To You? This really is quite sad...
I've always wondered why in Touhou works like these where one character gets old and dies to leave their immortal partner (ie Alice/Marisa, Mokou/Keine, Yukari/Reimu, etc) that they just can't come back as a ghost to see them, though. Maybe it's because the souls of the humans aren't really allowed to come back as they please, and the ones that are ghosts are the exceptions to that for some reason? I dunno...