Saber of Black

One of the Servants of the “Black” camp. His true name is Siegfried. He is an undisputed great hero and “dragon slayer” who appears in the “Nibelungenlied.” His tall stature, slender body and tanned-colored skin are proof of him having been soaked in the blood of a dragon. The reason the weak point on his back is exposed is because of a “curse” that forces him to do so. It conceptually prevents him from protecting his back. Even if Siegfried were to be protected by something like a spherical barrier, a hole in it would definitely open facing his back. However, there aren't many heroes who possess the skill to pull off the tricky feat of aiming for Siegfried's back while fighting him in the first place.
Just as the impression of his face gives off, he is reticent, but his bearing, which overflows with dignity, makes him seem well-bred. He is the type to talk with a simple, unsophisticated tone and hold back his words to the absolute minimum needed. That is the foundation of his personality, and it deepened the antagonism between him and his Master Gordes.
Siegfried's legend came into existence around the 5~6th century and spread to many lands. At nearly the same time as the “Nibelungenlied,” the story of the hero Sigurd, the “Volsunga Saga,” came into existence. Wagner's opera “Der Ring des Nibelungen” is a masterpiece work that compiles these legends together.
In the light novels, Siegfried as he was portrayed in the “Nibelungenlied” forms the basic essentials of his character, and he only knows about the Norse Valkyries from second-hand knowledge.
With him possessing the noble lineage of royalty, being known for various his adventures and heroic tales and having met a tragic end, there are few heroes who are as heroic as him.
But it is precisely because he is such a heroic hero that he was unconsciously burdened by binds. A hero acts out of the supplications of the people, and therefore should not act without being asked. This is because a hero is such a being. Because they possess overwhelmingly immense power, they must not act out of their own will and try to grant their own wish. A hero is a being that fulfills the wishes of others, and must not ever tread any further than that.
When his wife and the wife of his brother-in-law wounded each other's honor and a collision between them became inevitable, he once more granted everyone's wish. He thought that it would be resolved if he, the cause of it all, died. In the end, it called forth even more tragedy, so it seems the curse of the Rhine gold took effect perfectly.
As a Servant, he is basically hard, solid and tough. “Armor of Fafnir: Blood Armor of the Evil Dragon” blocks all regular attacks up to B rank and all Noble Phantasm attacks up to B+ rank and, in addition, “Balmung: Phantasmal Greatsword, Felling of the Sky Demon” unleashes a wide-range attack that displays its true worth against an army. The blue jewel encased in the hilt of the sword contains mana from the Age of Gods, and it is possible to pull out the sword's power to the maximum with just small quantity of it. During his battle with Karna, Sieg managed to use the Noble Phantasm so many times consecutively only because he continued instantaneously pulling out the sword's power while replenishing his mana with “Galvanism.” The Servant version of Siegfried cannot instantly replenish the mana he expends, so his sword's accumulation of mana is overwhelmingly faster than that of an average Anti-Army Noble Phantasm, though still not as fast as Sieg.
In the initial plans of the Yggdmillennia team, Siegfried would go out to the front lines and receive the attacks of enemy Servants as Vlad III's protector. Vlad would be positioned to control the battlefield with “Kaziklu Bey: Lord of Execution.” Jack the Ripper would kill the enemy Masters and cause chaos in the enemy's rear guard along with Frankenstein. Astolfo would guide the golems and control the situation with Avicebron, while Chiron would act as support by shooting at areas where a hole in the enemy lines could be made.
It was truly a perfect formation as an academic theory, but they were forced to tearfully change tactics due to Siegfried unbelievably leaving the battlefield before the war actually began.

Fate/Apocrypha material: Fate/Apocrypha Encylopedia