Ushiwakamaru vs. Benkei
Besides the tengu legend (one day I'll make a long post about it), this IS the other source of inspiration for Waka's character design, and the reason why he carries a flute. There are even some sketches in the artbook of a previous version with a shirt over his head. It's hard to get this amount of info while googling Ushiwakamaru/Yoshitsune. In the end, it was searching for Benkei what did it.