If you go to Oku Hanako’s official site, you’ll be greeted with a pop-up message:
There’s a music score in the background, and being the musical guy I am, I just had to figure out what it is. You can probably guess, but after playing through it in my head, I concluded that it’s a snippet of Kusabi / 楔, specifically the end of the pre-chorus until a little after the end of the chorus (the chorus starts at [C]).
No wonder I haven’t been able to work out the chords by ear. I knew the chords, which are written above the staves, were somewhat complex, but I didn’t think they were to this degree. You have major chords, minor chords, slash chords, sus4, add9 (both major and minor), and even some combinations of them. Don’t worry if you understood nothing in this paragraph: just know that it’s appreciatively complex.