Erna(エルナ, Eruna) is one of Subaru's classmates from the Steel Saint training. As he is aware that he, like all Steel Saints, does not have a Guardian Constellation, Erna has drawn a star on his Steel Cloth instead to give him the same hope of protection.
During the war, the training center Steel Saints is attacked by Pallasites and flees Erna with his friends and teachers, finding refuge in a temporary camp.When Koga and Subaru are attacked by Daggers Hati and Pallasites Soldiers in the ruins of the center. Erna and several Steel Saints return to the scene to assist. The enemy has already left, but they can still drive their hand and Koga in their new headquarters.
Shortly after, the base is attacked by Hati and Erna his troops took part in the battle. When life Subaru is in danger, Erna intervenes to save his friend and killed by Hati. Before making his last breath in the arms of his classmate, he said that although he did finally could not find its protective constellation, he is convinced that his companion will be able to.
- He name was based on asteroid.