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 for Steel Saints is attacked by Pallasites and Erna flees with his friends and teachers, finding refuge in a temporary camp. When Pegasus Kōga 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.
Soon the base is attacked by Hati, and Erna took part in the battle. When Subaru is in mortal 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 could not find his Saint constellation, he is convinced that his companion will be able to.
- His name was based on asteroid.