"And the black-clad ones could hover, in vision, but not registering in the slightest, until one thought to look for them. While this lead to common depiction of ninja, it also could be an influence from the Walking Willow. A more concrete one within Japanese culture, of course, is the Noppera-bō, or faceless ghost.
(Picture in page, same one taken here from Wikipedia - Simon.)
A faceless being, depicted here in fine clothing. The similarity should be obvious, and even to this day, sightings persist. The Japanese concept of the ghosts of people enacting their grudges on the living seems to be a rather close fit for the Willow.
He has been here a very long time. And I fear that He will be here for a very long time to come."