Kym was taken to the Eyrie as a ward of Rumar Arryn at the age of three. There he grew up with the third-born son of Lord Arryn, Dake Arryn. They became good friends, and played and trained a lot with each other. Kym later turned out to be a gifted swordsman.
Kym Egen and Dake Arryn travelled through the Seven Kingdoms, having both been knighted.
Kym maintained his position at the Eyrie, even after Dake was removed as castellan, having proved himself highly capable in the role.
Nathaniel Arryn restarted the Vale's historic knightly order, the Brotherhood of Winged Knights in 510 AC. Kym was named the new commander of the order, being chosen over Addam Belmore by Nathaniel personally.
At Nathaniel Arryn's order, Kym began training Theon Arryn with sword and lance, often times along side his own squire, Hugo Templeton. Following word of rebellion in the Three Sisters, Kym marched with the Arryn host and continued to train Theon on the road. At Sisterton, Kym commanded the rearguard of the Vale's assault alongside five of his fellow Winged Knights. He was also tasked with protecting Theon, who joined them in the battle as Lord Arryn under Nathaniel's command. He was forced to subdue a sisterman during the battle, when the man attempted to attack Theon and his squire.
Following the Sacking of Sisterton and the injuries sustained by Nathaniel Arryn, Kym tried to assist Theon by taking him under his wing and shielding him from the more ambitious lords of the Vale who attempted to take advantage of the young lord's situation. He stood by Theon's side, constantly offering him his unwavering support as he began to take the rulership of the Vale into his own hands, and eventually feeling an almost paternal care for him. When Theon formed a new Lords Declarant to assist him in the war, Kym proved he was loyal to Theon and not the ruling council, when he continued to do as the young lord requested, instead of following the wishes of the Lords Declarant.
Following the end of Sunderland's Rebellion, Kym acted as Theon's right hand. As the Vale army sailed for Gulltown, he assisted Theon in the task of getting Nathaniel to White Harbor, where he could receive the care he needed for his injuries. Once in the city, the Arryns stayed in New Castle with Lord Androw Manderly, an old friend of Theon and his mother. During their stay, the Starks arrived in the city to call upon Androw. Kym and his fellow Winged Knights were the first to catch wind of the arrival and the subsequent battle that took place in the streets between Androw's forces and Jojen Stark. He rushed the news to Theon and a now recovering Nathaniel, who ordered the Winged Knights to take Theon to the gates of New Castle, and discover the truth of what was happening. Kym did as he was commanded, and met Androw's brother and heir, Cerrick at the closed and barred gates. A confrontation between the Winged Knights and Cerrick Manderly issued, despite Theon's protests. and the gates were finally opened after Kym was forced to take hold of the Manderly and subdued him before he could order his men to attack. The Starks entered the castle and executed Androw for his crimes on the spot. When they discovered Cerrick also planned attacks against their family whilst in the city, Kym was the first to denounce Cerrick for his crimes. He and Dickon Lipps seized Cerrick as Lord Stark sentenced him to die as well, ordering his direwolves to kill the man.