Tapu Koko is the Guardian Deity of Melemele Island. He is also been referred to as a God, given that he watches people when no one can see him.
Tapu Koko first appeared in "For a God's Sake", where he watches Ash throughout his time in the Alola region. He confronts Ash who mistakes him for mosquito and tries to kill him with insecticide. Lillie, Mallow and Lana explain that he is a God and has seen most inappropriate things Ash does. He then lures Ash into the woods to have a battle with him, testing his new Z-Moves. Ash and Pikachu then use their Electrium Z on Tapu Koko, only to have him disappear.