When watching episode 10.5 of Kekkai Sensen, I found that Black, when possessed by King of Despair, was whistling some tune. I'm sure that Black whistles that tune more than once.
I remember hearing it:
- Before Leo meets Black in a train, and
- When Black is waiting for White to wake up to ask for taking Leo's eye.
Is it from another song? Or is it an original tune?