Depending on the backstory Jair is the last of the Line of Kings, or the first of the new line.

From the NES manual;

As the last of the Line of Kings, only you possess the power to prevent the evil Warlock Lord from raising the Behemoth and unleashing it upon the land. Your quest lies within the Castle Shadowgate with its secret passageways, collapsing floors, and armed creatures. Your courage and wisdom will see youthrough... but a good sword might be helpful.

From the Beyond Shadowgate description;

As the last descendant in the "Line of Kings", prince Erik is summoned home to regain his rightful place on his father's throne. But alas, Prince Erik is accused of his father's murder by the evil Royal Advisor Belezaar and is thrown into the dungeon to be executed at dawn!