The Healer was once a living man, but he died in a forest fire accident while out picking mushrooms. His soul was reborn courtesy of the mycelium network his body lay rest with. The Healer still had his human mind and consciousness. And he used his unique powers to help the living.
The Healer could commune with the trees, and they would tell him what was wrong with the living. Then he would use his own undead body to heal them, by absorbing their pain and suffering onto himself.
The Healer was a true friend to the living, and they loved and trusted him. But one day, a group of humans came to the forest where The Healer lived, and they started burning down the trees.
The Healer tried to stop them, but they wouldn't listen. They just kept burning, until all the trees were gone.
Without the trees, The Healer could no longer help the living. And so he left the forest, never to return.