Damon is the Gym Leader of Crestlight City's Gym, known officially as the Crestlight Gym. He specializes in Dark-type Pokémon. He gives the Crescent Badge to trainers who defeat him. He is knowed as the night wanderer
Damon appears at first in Blackleaf Woods, where he is training with his Pokémon in front of a lost house. He's telling you, that he will go back to his gym, so can battle him. He is healing your Pokémon and then he disappears.
After this, He appears as the leader of Crestlight City's Gym. This gym is the second to be challenged by the player. Trainers who defeat Damon will receive the Crescent Badge, along with TM98 Dark Matter.