Memory Loss (Fate Apocrypha Mordred X OC)

He appeared right after the holy grail war, right after the castle collapse, in the middle of the ruins. Clueless of his existence. But that didn't bother him. He wandered and found a man limp injured on the ground, still breathing. For some unknown reason, he took care of the man until he was able to walk again.

The man's name was Kairi Sisigou, survivor of the last war. And since then, they became close friends.

Years has passed since. Sisigou filled him everything about the current era and the holy grail. Then, another war started randomly out of the blue. One which nobody has heard of. With the grail in their hands, it's up to them to protect it at all cost.