Saber was born with the prana of a dragon, shown by the “Pendragon” name, as Uther ordered Merlin to grant his successor the factor of the dragon (竜の因子[1]), She is the holder of the Element of the Red Dragon who carries the blood of a phantasm in a human body, granting her a heart and unique Magic Circuits that transcend what can be achieved by normal humans and magi. Saber's Circuits are called a Magical Core, which is a fundamentally different from those of a normal magus. While a magus' body can be considered a machine which creates magical energy, hers acts as a factory which creates magical energy. It functions as an enormous prana core with tremendous capacity that generates energy independent from her body, but it will not function without the proper energy to act as a "starting key." While she is almost out of energy in Fate after using Excalibur, she has a great amount of undiluted magical energy within the core, but it is meaningless without enough energy to start it. It requires a "fire" to burn in order to activate it, and it doesn't matter how small it is, as it will resume activity once a path to send magical energy is connected. During Realta Nua, Shirou uses part of his own Circuits to start it, and encounters a dragon that takes form in Saber's unconsciousness. Shirou calls it "an absurd 'true nature' that is appropriate for the best Servant." Once reactivated, it shows its nature as a prismatic circuit as it glows with all the colors of the rainbow and burns with a golden fire as its circulation restarts.[2] Due to the characteristics of Dragon, she has much trouble with existences such as Siegfried, who is associated with tales of “Dragon extermination”, and weapons with an anti-dragon attribute such as Arondight.
Saber's dragon blood grants her a very strong Magic Resistance capable of resisting a Command Spell. It is possible it will not function to its fullest extent if she is targeted by “Dragon-slaying Sorceries.”[3]