Qin Zongquan
It is not known when Cheng Rui was born, but it is known that he was from Qing Prefecture (青州, in modern Weifang, Shandong). In his youth, he had once killed another person when drunk, and thereafter fled from his home prefecture in order to avoid vengeance. He became a Buddhist monk for a while, but later joined as a soldier under Tang rebel general Qin Zongquan at Cai Prefecture (蔡州, in modern Zhumadian, Henan). He was adopted by one of Qin's generals and thereafter changed his name to Guo Yu.