King Stannis Baratheon, sometimes refered to as "The Mannis", is the King in the Narrow Sea and Lord of Dragonstone. He is a claimant of the Iron Throne and will eventually attempt to push this claim.
Stannis was given Dragonstone and the two islands of Claw Isle and Driftmark as vassals by Robert Baratheon upon taking the Iron Throne from the Targaryens. Stannis took this as a slight as the Baratheon's much more powerful homeland was given to his Younger Brother. Historically though Dragonstone is given to the expected heir of the Iron throne so this may have been a silent acknowledgement of Stannis being the best heir. Years later Stannis, as master of ships under Robert, led the Iron Throne's fleet when the Ironborn, under Balon Greyjoy, rebelled. Stannis defeated the Ironborn's fleet under the command of Victarion Greyjoy near Fair Castle and then restored Law and Order back to the Islands.