Douglas is a playable character in Fire Emblem: The Binding Blade. He is the Great General of Etruria, and is very loyal to his country. Douglas is also the adoptive father of Larum. He first appears as an enemy, but if he is left alone he will become a playable character in the next chapter, and you will access the gaiden chapter for that level. Douglas was the first person to know that Elffin was truly prince Mildain and helped him escape to the western Isles with Larum, on the ship owned by Geese. After the war, he retired to live a peaceful life, with Perceval succeeding him as Great General.
In Chapter 16, the enemy Douglas will not attack Larum or Elffin. Either can talk with him, but he will remain an enemy.