As a servant of Shishigou Kairi in the Red Faction during the Turifas Holy Grail War of Fate/Apocrypha, Mordred is an identity who serves as the "son" of King Arthur but is actually female. She was raised secretly as the male heir to the throne and instructed to conceal her true gender while obeying the king's orders.
She was conceived between Arturia and her sister, Morgan le Fay, through unorthodox circumstances. Arturia, normally female, was a pseudo-male at the time due to Merlins magecraft, so Morgan enchanted her with a spell to extract sperm from her. Morgan developed it within her own ovary, and made the child into a homunculus clone of Arturia. Due to her status as a homunculus, she was given a much faster growth period than a regular human being, and her life span was far shorter.