In Pokémon Sun and Moon, a Trial Captain is the highest ranking member of a trial of The Right of the Island Challenges. Each trial captain is assigned in different parts of the Alola region. They will apparently be replacing Gym Leaders in the Alola region.
List of Trial Captains
Mallow
A Trial Captain who specializes in Grass-type Pokémon.
Lana
A Trial Captain who specializes in Water-type Pokémon.
Sophocles
A Trial Captain who specializes in Electric-type Pokémon.
Kiawe
A Trial Captain who specializes in Fire-type Pokémon.