I hardly think it’s a coincidence that there is a music titled “Ushio” in a game where one of the most characters is named “Ushio”, xD. Key just doesn’t do coincidence.
I have to admit, I didn’t read most of the VN. I discovered it with the anime, lol. In the anime, as far as I recall, that music mainly plays during After Story (well, it’s also during the “sponsor title”, but I hardly consider it part of the episodes xD). There is also the song version of this theme (Komorebi), its lyrics are pretty much about Ushio.
That said, you’re totally right when you say a character’s theme usually plays when the character is on-screen. It’s kind of a VN standard. Looking at it this way, Ushio wouldn’t have her own theme. Then again, it just doesn’t look like Key to create a character and not give it a theme.
By the way, do main characters (I mean the boys) have their own themes? I always figured “Town, Flow of Time, People” was Tomoya’s, but I have no idea about the others.
