Nathaniel Arryn, Theon's uncle, held regency until he came of age, and raised him in the Eyire following his parents' deaths.
Theon has a stutter.
Theon returned to the Eyrie with his mother, Lyanna Stark, and his father's Arryn kin. While there, Lyanna began planning in secret to return to Winterfell and take Theon with her.
Theon traveled from the Eyrie to King's Landing with his mother, Lyanna, to attend the Tournament of the Hand in honor of his uncle's promotion to the highest seat on the small council. Once departed from King's Landing, Lyanna brought Theon to Winterfell instead of the Eyrie as she had planned.
In the North, Theon grew up alongside his cousins and mother's kin. Lyanna's close friend and eventual lover, Androw Manderly, became a father figure for Theon during these years.
Theon was commanded to be returned to the Eyrie by Queen Danae Targaryen following the death of Thaddius Lannister. Meanwhile, Lyanna was forced to wed Olyvar Bolton by the crown, despite having secretly wed Androw before a heartree, with a septon and Theon as witness. This was the first and last time Theon had been apart from his mother. Upon returning to his ancestral home, he began being fostered by his uncle, Nathaniel Arryn. Lyanna died not long after taking up residence in the Dreadfort, but not before granting Theon a half-brother, Warne.
Theon settled into his new life in the Vale quickly once finally past the shock of being separated from his mother. He became friends with his elder bastard cousin, Mya Rivers, as well as closer to the rest of his paternal kin. Theon eventually traveled south once more with his uncle Nathaniel, during the Trial of Symeon Stark, his other uncle. Theon was eager to travel by sea and witness King's Landing for himself, having not recalled the trip when he was much younger.
Upon returning to the Eyrie once more, Nathaniel Arryn began work to restart the knightly order, the Brotherhood of Winged Knights, in order to ensure the protection of Theon. Following its formation, Theon began training with sword and shield at his uncle's insistence, being taught by his new Commander of the Winged Knights, Ser Kym Egen.
During Winter, word of rebellion in the Three Sisters, reached the Arryns. Nathaniel used this as an opportunity to introduce Theon to his realm and teach him the ways of lordship during wartime.
Theon continued his training with Ser Egen and his squire, Hugo Templeton. He began idolizing the slightly older squire and attempted to begin a friendship with him. In the Sacking of Sisterton, Theon was under the protection of the Winged Knights as they led the rearguard. During a scuffle with a group of sistermen, Theon witnessed Hugo get into danger. He charged at the man attacking Hugo and helped in saving him by distracting the man long enough for Ser Egen to come to both their rescues.
Following the battle, Theon received word of Nathaniel's injuries, and spent much of his time by his uncle's bedside before taking up the task of leading the Valemen in ending Sunderland's Rebellion, alongside a group of hand-picked Lords Declarant. Queen Danae descended on Sisterton not long after the sacking in search of the rebel king, Elys Sunderland, and Theon met her in his uncle's place. She granted Theon and his council her leave to deal justice out on the rebels and end the war while she continued her search. They did so, and Theon held his first court in Sisterton shortly thereafter- where he surprised many present, including members of his Lords Declarant, by proving he may not be a completely incompetent lord after all. Amongst the decisions made during the court were Theon sending the sistermen fleet captains to the Wall, revoking the lordships of Borrel and Longthorpe, naming them landed knights instead, and stripping all titles and holding from House Sunderland, naming Petyr Arryn of Gulltown the castellan of the castle and islands.
His uncle's condition worsened over the weeks to follow the rebellion, and after the Knights of the Vale sailed home, Theon ordered his flagship sail for White Harbor, the nearest city, to find Nathaniel the care he needed. He was reunited with Lord Androw Manderly, who welcomed the Arryns with seemingly open arms. Theon spent several days with the Manderly household as he awaited for his uncle to wake, all the while rekindling his relationship with Androw. The day Nathaniel finally woke up, word reached New Castle that Jojen Stark and his own household had arrived in White Harbor. Theon's initial excitement to have his family reunited was quickly dashed when it was discovered by his Winged Knights that the visit was not for the Arryns, but for Androw. Ser Egen told Theon and his uncle of the battle that occurred in the streets between the Starks and Manderlys, and of the barred gates. Nathaniel then ordered Kym and Theon to go to the gates and demand they be opened, instructing them to find Androw and get to the bottom of hat had happened. They did so, and in the yards of New Castle, Theon watched in horror as Kym and Cerrick Manderly, Androw's brother, grew hostile. A battle between the Winged Knights and Manderly men was threatened before Kym ceased Cerrick and held him until the castellans of the castle relented. The gates opened for the Starks and Androw, who was their prisoner. Jojen Stark was quick to begin his execution of Androw, despite Theon's pleas.
Theon's sixteenth nameday was only days after Androw's execution. He became the ruler of the Vale and Lord of the Eyrie however was deep in mourning for his former father figure. His uncles arranged for Theon's half-brother, Warne, to return with Theon to the Vale as his ward. It was revealed that he was in fact the son of Androw and not Olyvar Bolton. Theon, recalling his mother's wedding, realized that made Warne the rightful heir to White Harbor. Jojen agreed and it was decided Warne would learn from Theon for five years before returning to White Harbor and his father's kin. Warne assisted Theon in beginning to move on from Androw's death.
James’ face was beginning to fade in her mind, but whenever she looked at Theon, she could see him again. - thoughts of Elyssa Arryn
“Your uncle never mentioned that you were simple. You’re not simple, are you?" - Danae Targaryen
"He's smart and did what was right." - Mya Rivers
Gods, what a stupid smile. - thoughts of Cerrick Manderly
His lordship was such a dutiful boy, and growing into a diligent man. - thoughts of Kym Egen
The young lord, in that moment, looked more like his father than Jojen remembered him. - thoughts of Jojen Stark