History
Theon was born a Greyjoy, one of several sons of Balon Greyjoy, lord of the Iron Islands. But when his father mounted a failed rebellion against the Crown, Theon was sent to live at Winterfell, where he grew up among the Starks. That duel identity would prove tricky for him later in life when he was forced to choose between remaining loyal to his birth family or his adopted one. He chose Option #1, and his life has basically been a disaster ever since.
After a failed bid to claim Winterfell for the Iron Islands, Theon was imprisoned and tortured by the sadistic Ramsay Snow. The torture was so bad that Theon shed his name and adopted the personality of the servile, sniveling Rook, an identity he kept until Sansa Stark married Ramsay and brought him slightly back to his senses.
Theon helped Sansa escape Ramsay, and then took his leave of her to return to the Iron Islands, where he vowed to support his sister Yara’s claim to the Salt Throne. Their crazy Uncle Euron won the throne instead, and Theon joined Yara in stealing a bunch of ships and sailing to Meereen to make an alliance with this Daenerys Targaryen person everyone is always talking about.
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Status
Theon stood by Daenerys and Yara as they sailed towards Westeros. Yara intends to defeat their uncle and take the Iron Islands for herself, which means Theon will probably see battle in season 7. Has he shaken off Reek to the point where he can handle that?