Okay, this has been eluding me for years and I'd just forget about it over and over again. But somehow I felt like looking up the endings for this game and heard it again, and I'm captivated all over again. Somehow just has to know what the name of this song is.
It's the track that starts at 1:32 or so. So fuckin' good. Some people in the comments say it's a special version of "Your Rain" that's not on any of the OST's. I can't tell if it's all supposed to be one track that just fades into another part, or if the track that starts at 1:32 is separate from the actual a capella version of Your Rain at the end.