In your usual Holy Grail War, it is very likely that he will be summoned under the Extra Class 「Gunner」.
Billy the Kid. His real name is William Henry McCarty Junior. A representative outlaw from the American Wild West, even in modern times his popularity is extremely high. From the fact that one of his few remaining pictures was put for a price of two hundred million yen in an auction, one can perceive said popularity. His father is unknown, but it seems that he received a higher education from his mother and, considering he was a Western outlaw, he left behind some skillfully written letters.