People in Type-Moon have a hero simulator.

Fujimaru Kyouya, 17 years old, is a transmigrator who has loaded a hero simulator.
You were born. You are a boy.
Your family appears to be a fallen knightly clan. Your father teaches you swordsmanship, hoping you can pull the sword from the stone, become King of Britain, and defeat the lowly King Vortigern.
You did not pull the sword from the stone.
……
You have learned the truth about Britain, and you have joined Vortigern's forces.
……
He was a formidable adversary that the Knights of the Round Table could not afford to ignore;
He was the assassin who swung his sword at Gilgamesh;
He was an old friend of Miyamoto Musashi, both an enemy and a friend;
He was the successor recognized by King Hassan;
……
"I'm just an average high school student."
During the Fifth Holy Grail War, Fujimaru Kyoya, wielding the Holy Sword, spouted nonsense with a straight face.