Nanu is a character introduced in Pokemon Sun and Moon. He is the Kahuna of Ula'ula Island. He specializes in Dark-type Pokémon. He grants the Ula'ula stamp to Trainers who defeat him.
But in the Legacy, that he was revealed that he was the true leader of Team Skull, because Guzma was his second banana and would plan to take over Melemele Island as well, but doesn't know that Lusamine is behind the island domination, because she has the Ultra Beasts, under the King of Pokelantis' commands.
And he is voiced by Mark Hamill in the English dub and he is reprised by Masaki Aizawa in the Japanese dub.