Den I. Koopa is one of Bowser's 9 children, and the oldest of them. He was the second-in-command of the Koopa Troop until he lost the Battle of Grass Land. Instead of living with his dad, he ran away to Ocean Land where the animals befriended him and made him their leader. However, Den still liked the Koopa Troop a little, which is proven in Super Mario Bros. 4 where he is the boss of World 2.
He first appeared in Back in Time, Bowser when after Bowser changed time, Den was the only child born, and Bowser became friends with him instead of enemies. However, Den is soon killed by Luigi in Bowser's Castle, so once again Bowser goes back in time to his dad's era to make sure his son is never born. He succeeds, but then Father Time comes and brings time back to normal, and Den Koopa is brought back to life.
After this appearence, however, he also appeared in Super Mario Bros. 4. He was not put into the remake.