Zoonkie

Noonkey lives in a deep cave  under Equestria. Her job is to water the tree of life, with her tears and only her, can do so. The tree of life connects to the core of the earth, creating life for all organism and plant life.

    If ever the tree dies, the whole planet dies with everyone in it, she lives a lonely life, but as her tree and her tears to keep her by. Protected by the Nobel guardian Halo that does all the dirty work, like violence and she does the crying.

Noonkey can never leave the caves, she must stay and water the tree for centuries to come and so on and so on. Halo wouldn't allow it either way, halo will scare off any trespassers that go in the caves, no one can find out about the tree of life or Noonkey and her ability to heal wounds with a single tear.