Who is mr x billups?

"He's a funny man," said Jem. "X's his name, not his initial. He was in court one time and they asked him his name. He said X Billups. Clerk asked him to spell it and he said X. Asked him again and he said X. They kept at it till he wrote X on a sheet of paper and held it up for everybody to see. They asked him where he got his name and he said that's the way his folks signed him up when he was born."