The King does very little all considered. He is only really notable in that he is the employer of The Guard, and in SPO and SP7 he dies during the Slime Uprising arc. He might not die during SP7, though, but he does appear; he might not die because The Guard is still acting as a guard at the time of "Clone 732".
The King was king over all people in PC. He was killed during the Slimes' attack to the worlds.