Symeon Baratheon was Lord of Storm's End, Lord Paramount of the Stormlands, and head of House Baratheon. He was the father of Lyonel Baratheon and great-great-grandfather of Robert, Stannis and Renly Baratheon.
He is mentioned in Lyonel Baratheon's entry in The Lineages and Histories of the Great Houses of the Seven Kingdoms. Symeon's wife is listed as Meera Baratheon, it is unknown which house she originally belonged to.
In the books
In the A Song of Ice and Fire novels, there is no such character.