Theon Greyjoy was the youngest son of Balon Greyjoy, lord of the iron islands when Theon was very young Balon rebelled against King Robert Baratheon starting the war which claimed the lives of Theon's two elder brothers Rodrick and Maron. When Balon lost the war Robert punished him by Making him give Theon up as a hostage to Ned Stark in Winterfell.
Despite his status as a prisoner Theon bonded with the starks and became best friends with Ned's son Robb, however Theon never forgot his place and felt he'd never be truly accepted, in order to cover these insecurities Theon put on a display of arrogance and spent time with prostitutes in order to make himself feel more powerful his favorite one being Ros.
On their way back from an execution Ned, his bastard Jon Snow Theon and Robb Came come across 5 dire wolf pups who's mother had died and Ned ordered for them to be given a quick death. When Theon cavalierly moved to carry out Ned's orders himself it noticeably upset Robb's brother Bran, thus angering Robb.
Jon persuaded Ned to spare the pups and let his 5 children take care of them. When Jon found a sixth pup Theon mockingly declared it the runt of the litter and told Jon it belonged to him, simultaneously insulting Jon for his bastard status.
After the attempted assassinations of Bran, Theon was told by Ned's wife Catelyn that she suspected the lannisters of being involved. Theon and Robb both wished to seek revenge through military action but were counseled by Master Luwin to be patient. When Catelyn left with Ser Rodrick Cassel to look for evidence Robb was left as lord of Winterfell. When Tyrion Lannister visited on behalf of Jon to design a saddle for Bran Robb treated him with suspicion. As Tyrion was leaving Theon condescendingly told him to ask for Ros and did not take it well when Tyrion retaliated by mocking Him for being such a loyal captive.
When Tyrion was taken prisoner by Catelyn his brother Jaime retaliated by attacking Ned and killing Several of His Men. Theon Was outraged by this and advised Robb to go to war with the Lannisters reminding him of his duty to House Stark only for Robb to throw it back in Theon's face and remind him he wasn't a true Stark. Moments after this when Bran was taken prisoner by Wildlings Theon saved his life by shooting their leader Stiv in the back with an arrow, though this upset Robb who felt Theon had put Bran's life in unnecessary danger. After saying goodbye to Ros who was leaving for king's landing and sincerely telling her he was going to miss her.
Theon was left in a foul mood and chose to take his aggression out on Osha (the wildling prisoner Robb took) demanding she call him Lord. However Theon was warned off by Luwin who reminded him that he of all People should Know the importance of treating prisoners as guests. When Robb called his banners to war following Ned's arrest, he got into an argument with Greatjon Umber. As Greatjon began to draw his weapon Theon quickly moved from his chair to defend Robb, however Robb's dire wolf Greywind darted across the table and bit off two of Greatjon's fingers. Robb mercifully offered the Greatjon his weapon to cut his meat for him. Greatjon called the meat bloody tough, causing laughter from everyone including Theon.
Theon fought alongside Robb during the battle of the Whispering Wood in which Jaime was taken prisoner. While Theon voiced his desire to see Jaime dead Robb refused as it could have caused the lannisters to kill his sister Sansa and Arya in retaliation. When Robb was proclaimed kKing in the North, Theon was among the first to swear fealty to Robb. Theon reasoned with Robb that they could use Balon's fleet to attack the king's landing.
Robb was persuaded by this and sent Theon to pyke in order to obtain Greyjoy support. On the boat there Theon seduced the daughter of the ship's captain and excitedly told her of the warm welcome he was expecting from his family and other ironborn. When he arrived Theon met his sister Yara, but failed to recognize her as she was posing as a stranger in order to see what kind of man Theon had grown up to be. Theon flirted with her relentlessly and bragged of his own importance. When Theon met Balon his father treated him coldly, accusing him of forgetting his roots and becoming loyal to the starks.
Yara then revealed her deception much to Theon's disgust and Balon compared Theon unfavorably to his sister. In a later meeting Theon continued his efforts to sway Balon to Robb's side but Balon refused and accused the starks of making Theon weak. While Theon tried to point out how Balon's ambition was what cost them the chance to have a proper relationship, Balon was furious and struck his son across the face. Theon initially planned to warn Robb of his father's plans to attack the North but instead decided to join the second Greyjoy rebellion in a bid to win the acceptance of his father.
While Balon ordered him to raid fishing villages fellow ironbron Dagmer persuaded Theon to seize winterfell, which Theon accomplished by attacking torhen's square, luring the stark's garrison away and threatening Bran with the lives of winterfell's inhabitants. When the captured Ser Rodrick spat in Theon's face, Theon wished to punish him by having him imprisoned but Dagmer talked a reluctant Theon into sentencing Rodrick to death. Theon performed the execution himself and ignored Bran's pleas for Rodrick to be spared, though he was visibly upset by what he had done afterwards.
On Osha's suggestion Theon granted her freedom in exchange for sexual services. Unknown to Theon Osha was manipulating him in order to free the captives Bran and his brother Rickon. Theon was furious with his men for allowing them to escape when Black Lorren implied it was Theon's fault. Theon responded by giving Lorren a violent public beating. When Theon failed to locate Bran and Rickon he ordered Dagmer to kill two innocent peasant boys who'd been taken in by the farmer, then had their bodies burnt, presenting them as Bran and Rickon as a warning not to mess with him. As he did this Theon looked away in disgust with himself as opposed to Dagmer who Sadistically laughed at Luwin's pain.
Theon then sent word to Yara to bring 500 Men. When Yara arrived she came with only twenty men and attempted to persuade Theon to abandon his prize and return to the iron islands but Theon refused. Regretful of killing the peasants Theon asked Dagmer to pay compensation to the farmer but found this would be impossible when Dagmer told him he had already murdered the farmer and his wife. During the night while Winterfell was surrounded by Northmen Theon vented to Luwin over the resentment he felt for Ned because of his brothers' deaths. When Luwin tried telling Theon of the kindness Ned showed him Theon responded by tearfully asking Luwin "you know what it's like to be told how lucky you are to be someone's prisoner, to be told How Much You Owe them and then to go back home to your real father". Assuming a more sympathetic tone Luwin advised Theon to atone by joining the Night's Watch but Theon rejected the idea as he was terrified Jon would kill him to avenge his brothers' deaths. Luwin gently put his hand on Theon's shoulder, telling him "I've known you many years Theon Greyjoy, you're not the man you're pretending to be". Theon acknowledged the truth of this but felt he was already too far gone.
When Theon tried to rally his men for battle he was betrayed by Dagmer who turned him over to Roose Bolton, who had his men torture Theon. Roose's Bastard Ramsay Snow (posing as one of Yara's Men) saved Theon, who then remorsefully confessed his crimes and acknowledged Ned was more of a father than Balon ever was.
When armed men entered Ramsay betrayed Theon, lying about him being responsible for the death of the men's comrades and soon, Theon was confined to another torture Chamber. Ramsay took great joy in abusing Theon, castrating him and conditioning him into being a loyal Servant named Reek. In order to demonstrate Theon's submissiveness to his father, Ramsay had Reek shave him while he gloated over Roose's murder of Robb. While Theon was deeply saddened by this and was on the verge of killing Ramsay, he ultimately was too scared of potential repercussions.
Theon then sadly told his captors that Bran and Rickon may be with Jon at Castle Black, which resulted in Roose sending Locke after the Starks. When Yara came to rescue her brother, Theon refused her help, insisting that he was Reek. Ramsay then sent Reek to enter Moat Caitlin under the guise of Theon. Reek promised the ironborn safe passage home if they peacefully surrendered Moat Caitlin, however Ramsay had the ironborn killed anyway and told Theon he never had any intention of Sparing them.
When Sansa became a prisoner of House Bolton she begged for Theon to help her escape, instead he reluctantly informed Ramsay of her plan to spare her and himself more pain should the plan fail. When Sansa finally succeeded in escaping she was confronted by Theon and Ramsay's girlfriend Myranda. After hearing Myranda taunt Sansa over how she planned to castrate her Theon chose to kill Myranda and help Sansa escape. After escaping and almost risking Ramsay's torture to keep Sansa safe as well as saving Podrik Payne's life Theon retuned to the iron islands and gave up his claim to the Salt throne in order to support Yara, successfully rallying supporters for her by his speech at the King's moot, however Yara was challenged by their Uncle Euron who had killed Balon and revealed his plan to marry Daenerys Targaryen.
Euron then succeeded in being voted king, forcing Theon, Yara and her supporters to flee. Theon and Yara then met with Daenerys and Tyrion who was now acting as her advisor. Theon and Yara offered Daenerys 100 ships in exchange for eliminating Euron and Recognizing the independence of the iron Islands to which she agreed. When Yara's ship was attacked by Euron, Theon fought bravely, cutting down many of Euron's men. As Euron held Yara hostage Theon watched helplessly and jumped overboard, after seeing Euron's men torture and mutilate the defeated soldiers as it had set off his PTSD. When Theon was rescued from one of the few surviving ships loyal to Yara, he lied to it's captain Harag about trying to save her, leaving Harag unimpressed. Theon then returned to Dragonstone where he was met with a frosty reception from Jon, who was still angry at him for his betrayal. Nonetheless Theon accompanied Jon and his group to meet with Cersei Lannister. When Euron attempted to force Theon to swear fealty in order to save Yara Theon refused.
Theon was then finally able to least partially reconcile with Jon who told him he was a Stark and encouraged him to save Yara. Theon then attempted to rally Yara's remaining supporters but Harag refused, chiding Theon as a coward for abandoning Yara and attacking him. Theon then beat Harag to death finally earning the ironborn's respect. After saving his sister Theon went to Winterfell so he could join the Starks and finally make up for his betrayal, ultimately dying to save Bran from the Night King.