Mordred

Servant of Shishigou Kairi as part of the Red Faction in the Turifas Holy Grail War of Fate/Apocrypha.

Saber's identity is Mordred, the "son" of King Arthur, but she is actually female despite being raised as the secret male heir to the throne. She was told to hide her status and obey the king.

In the Fate/stay night anime, Mordred did appear in one of Saber's flashbacks.

(Source: Type-Moon Wiki)