Theon is the youngest son of Lord Balon Greyjoy and Lady Alannys Harlaw and heir to House Greyjoy in the Iron Islands. After his father's failed attempt of rebelion, he was taken as hostage by the forces of the Iron Throne and placed under Eddard Stark's care as his ward and squire to keep Balon in line from rebelling ever again. He has two older brothers ("Maron" and "Rodrik" in the books), both slain, and an older sister, Yara (Asha in the TV series). He is described as lean, dark and handsome youth of 19 years at the start of the series.
Theon is cocky, arrogant and smug, know for his poor treatment of most people around him. Even if he spent his late childhood and teenage years in the Stark household, he was always considered a outsider among Ned's children.
The "Game of Thrones" series has been adapted from the book of the same name, part of the series "A Song of Ice and Fire, written by George R.R. Martin. Theon Greyjoy first appeared in Game of Thrones #1.
During the War of the Five Kings, Robb sends him back to the Iron Islands to plead his father to form a alliance with the North against the Lannisters. However, Balon has other plans and decides to wage war on his own against the North. Having to choose between his adoptive family and his blood-bound family, he raids Winterfell to earn the respect of the Ironborn.
Theon Greyjoy first appeared in "Game of Thrones", a novel by George R.R.Martin. He has a POV in the second book "A Clash of Kings" and the fifth book "A Dance With Dragons".
Theon also appeared in the HBO television series, based on "A Song of Ice and Fire", titled "Game of Thrones". He is portrayed by Alfie Allen.