Saber's true name is Richard I (リチャード一世), the Lionheart (獅子心王). His bravery earned him the name "Lionheart" and even the enemy general praised him as the "greatest knight."

In the past, he, Saladin and the leading Hassan-i-Sabbah at the time fought together against a Dead Apostle during the Third Crusade. He is known as the "Wandering King," not because he was always headed off to a battlefield or on his way back from one, but because he was the king who wandered over the border between history and myth, the last king with one foot in an age when faeries and runes were still out in the open.

(Source: Type-Moon Wikia)