Saber (セイバー, Seibā?) is the Saber-class Servant of the Edelfelt sisters in the Third Holy Grail War. They were a Servant split into two separate entities due to the sisters' Sorcery Trait, Ore Scales.
The Heroic Spirit is one with such life experiences to allow for them to have two different alignments like King Arthur.
The Edelfelt sisters were considered one Master in the Third Holy Grail, but their unique Sorcery Trait, Ore Scales, allowed them to summon two sides of the same Heroic Spirit. The Sabers were together for a time, but later served separately under their respective Masters after they split apart. One made it to the final battle in keeping with the reputation of the Saber class, but the Holy Grail War ended without a victor due to the destruction of the Lesser Grail's vessel.
They appear in Fate/hollow ataraxia under the guise of Saber and her corrupted self, Saber Alter, due to Avenger being unable to accurately recreate them. Their Masters take on the appearance of Rin Tohsaka and Sakura Matou, and they quickly dispatch of Bazett Fraga McRemitz and Avenger during the first loop while they are out on reconnaissance.
The Sabers recreated by Avenger are strong in single combat. One easily beheads Avenger in an instant, while the other emotionlessly dispatches Bazett.