Answer
The character referred to as Saber of Red in the anime series "Fate/Apocrypha" is Mordred, the illegitimate child of King Arthur. Mordred is a powerful knight and wielder of the sword Clarent, and possesses the ability to draw out her master's potential as a master swordsman. Though initially appearing as an antagonist, Mordred's character develops throughout the series, showcasing both loyalty and a desire for recognition.