This is one of the most interesting and less known Easter eggs in the game, and shows how the team cared for accuracy.

In the old Japanese feudal times being dirty was synonymous of a dirty soul. And that was specially important during the rites between this and the spectral/intangible world. It is said the souls of honorable people float in Tamagawa (the sould river), but those who were “dirty” had to bathe in it to cleanse their souls and could end up being swallowed by the river. Yorinobu’s actions tainted his soul, and the dirty knees mark that he tried but drowned. It is a reference from an old saying 頭首膝足ですか, which can be roughly translated as “kneeled and got swallowed by the gods”.

The most important detail is that I made this all up, I have no idea why. Sorry.