Kahuna are individuals chosen by Alola's island deities to govern the islands of Alola.
From Pokémon Sun and Moon official website:
Each of Alola's four islands has a leader, called the island kahuna, who governs the island. Kahunas are chosen by the Pokémon known as guardian deities, which are also found on each island.
- Hala - The Kahuna of Melemele Island. He specializes in Fighting-type Pokémon.
- Olivia - The Kahuna of Akala Island. She specializes in Rock-type Pokémon.
- Nanu - The Kahuna of Ula'ula Island. He specializes in Dark-type Pokémon.
- Hapu - The Kahuna of Poni Island. She specializes in Ground-type Pokémon.