The Forest Spirit was once a regular Forest Ninja that literally became one with the forest — made up of everything organic around him.
Some Ninjas assumed the Forest Spirit was evil, until a wandering band of Wee Ninjas unwittingly entered the Dark Forest one afternoon. Only after being surrounded by a dozen Wee Devils, the Forest Spirit appeared from thin air ensnaring the evil doers and warning the Wee Ninjas not to enter the Forest again without proper training and guidance.
The Forest Spirit can see through the Darkness of the Forest, and carries magical leaves from a special tree hidden somewhere unknown in a remote region of Ninjatown.
“Do not enter the Dark Forest without proper training...”