King John is the King of England by his mothers working after the death of her elder son, Geoffrey. His claim to the throne is constantly disputed and thus England's security is vulnerable. He constantly stands behind the country of England and constantly defies the pope.
Prince Henry is King John's heir apparent son as long as John holds onto the throne. He plays a very minimal role in the play.
Arthur, The Duke of Britain, is a young boy who is the nephew of King John. As the pre adolescent son of the previous king, he is the natural successor to the throne and therefore a political target. If he attempts to go after King John a divided Britain might have a civil war to determine the succession of the English crown.
Queen Eleanor is King John's mother. Her two eldest sons Richard and Geoffrey are both dead. She uses John to assert her power behind the British throne.
Blanch of Spain is Eleanor's niece. Because she agrees to marry Lewis the Dauphin, she serves an important role in an alliance between France and England.
Lewis is the Dauphin of France, son to Philip. He marries Blanche, apparently achieving a union between France and England. He is very aggressive in his quest to take over the British throne and has been told by Pandulph to take advantage of the English "weaknesses" caused by King John's dealings with the Pope and with Arthur.
Philip is the King of France. he first supports Arthur and Constance in their quest for the throne, but then waivers to John's side. He switches again when John instigates a religious war against the Pope.
Constance is Arthur's devoted mother who spend the play trying to arrange a rebellion against King John to re assert her family's lineage with Arthur as king.
Hubert de Burgh is a trusted henchman of King John. He is loyal to every order given to him by John except the blinding and killing of Arthur.