Loyal and beloved retainer of Minamoto no Yoshitsune.

He is not the real Benkei, in fact his name is Hitachibou Kaison(常陸坊海尊) and he was nothing more than a servant to the real Benkei and Yoshitsune, barely mentioned in history. When his companions needed him most, however, in their final battle at Koromogawa no Tate, he fled cowardly in fear of losing his life. Overwhelmed with shame he trained in esoteric Buddhist practices, achieving limited immortality, took the name of Benkei and wandered the land trying to spread the legends of his former companions.

It's partially thanks to his efforts that the real Benkei is remembered as a folk hero.

In Fate/Grand Order: Absolute Demonic Front Babylonia

A warrior monk and Lancer-class Servant that Gilgamesh summoned to assist in the protection of Uruk's Northern Wall. Tales of Benkei's heroism and prowess as a fighter have spread all over Japan and much of the world, enduring even to this day. While he is a deadly warrior, Benkei also has a softer, pacifist side that hates battle of any kind