Theon Greyjoy was the fourth child of Lord Balon Greyjoy, ruler of the Iron Islands, and his wife Lady Alannys. He spent his youth as a ward fostered by Lord Eddard Stark, following the crown's victory over the Islands in the Greyjoy Rebellion.
After the Greyjoy Rebellion against Robert Baratheon was crushed, Balon Greyjoy was forced into fealty. He offered his son Theon toEddard Stark as a hostage, to ensure his loyalty to the throne. While living in Winterfell and practically being raised as a member of House Stark, he grew to become like a brother to the Stark children, especially with Robb Stark.
Theon served Robb Stark during the War of the Five Kings and fought alongside several battles with his foster brother. Theon was eventually sent to Pyke to treat with his father, Balon Greyjoy. His father refused Lord Stark's offer and commenced an invasion of the North instead, leaving Theon to return to Robb instead of serving in the war effort.
Theon would return to Riverrun to rejoin with Robb and tell him of his father's deception. Robb would eventually move North and repel the Ironborn in The Ironborn Invasion of the North. Theon would eventually become a very valuable asset to Robb in the future, as he is able to convince his sister, Asha Greyjoy to swear the failing Ironborn cause to the Kingdom of the North rather than Renly Baratheon.
Theon would live out his life in service of the Starks, never returning home to the Islands he so barely knew.