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 Darkness Badge to trainers who defeat him. He is known as the night wanderer.
In the games
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, if you can find his lost item in that house. 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 Darkness Badge, along with TM98 Dark Matter.
"Hello...I'm Damon, you were looking for me? ...A gym battle? Of course, but I need your help... You see, I left something in that house there... If you retrieve it for me, I will be happy to give you a gym battle!"
After finding his item
"You found my item, thank you so much, as promised, we will have a gym battle, I'll go there now, I will see you soon..."
"You made it here...good. As you know I am Damon, I specialize in Dark type Pokemon. Many people fear Dark Types, this is because they do not understand them, they can be magnificent! I shall show you in a gym battle, get ready!"
"You brought light in my darkness..."
After being defeated
"You have won this gym battle, congratulations! As your reward, you have earned this Darkness Badge. You also receive this gift. Its a special Dark type move that will cause damage and may confuse your foe. I wish you many victories along your journey (players name)!"