Yuber is a complete mystery. He is known as The Black Knight, but otherwise no one really knows who he is--or what he is. Actually, no one even knows wether Yuber is a he or a she. All that is known about him is that he owns a True Rune, the Eightfold Rune, which has unknown powers. Yuber himself possesses powers that are clearly beyond that of a human being.
Yuber is often seen in the midst of conflicts, such as during the Gate Rune Wars, Dunan Unification War, and the Second Fire Bringer War. He has been a subordinate of Windy for centuries though, helping her find True Runes and destroying countless lives in the process. He was hired by the Highland Army during the Dunan Unification War mainly due to Leon Silverberg who knew Yuber for some reason.
When Luc began to put together his plan to destroy the True Wind Rune, Yuber assisted him for the simple reason that the plan would cause mass chaos and death, which Yuber is fond of. In every conflict that he is a part of, when the going gets tough, he deserts to save his life, like a true mercenary.
Yuber harbors a great hatred towards other True Runes and actively tries to destroy whoever wields their power. The cause for this is unknown as well. Only Pesmerga knows more about Yuber. Pesmerga himself is also a mystery, but he says that Yuber is like a shadow.