Shougo Ryuujin is the son of Kengo Ryuujin and Chie Ryuujin, the younger brother of Tomoko Ryuujin and the childhood friend of Saho Shimatani.

He was a normal High-school student until he came into contact with the Dragon bone card who then became its adept as well as earth's keystone. In addition to that, he also has a black belt in karate.

(Source: Majin Bone Wiki)