A picture of Seung-ah at the age of 5 that her father still holds on to
Yoon Seung-ah was born in Saeloun Seoul, Reynif on August 28, 2537. When she was at the age of five, her father, a marine by the name of Yoon Hyeon-ki, left the planet to fight the Covenant, leaving Seung-ah to live only with her mother until the year 2545. While Seung-ah and her mother were enjoying family time in their home city's park, the Covenant invaded Reynif. Seung-ah's mother was killed by Covenant infantry, but not before she ordered her daughter to hide in the park's pavilion building.
Several hours later, Seung-ah was found by Petty OfficerDom-094, Sergeant Thomas Prado, and Lance Corporal Kylie Summers, who were also stranded in Saeloun Seoul. The SPARTAN called for evac, but a Phantom arrived at the park before any UNSC transport could, and deployed a team of six zealots. Dom-094 told Summers to take Seung-ah deeper into the building, while he and Prado would fight the elites. The SPARTAN was able to kill all six zealots, but suffered burns from a few plasma bolts. Prado, unfortunately, did not survive the attack. A D77-TC Pelican arrived soon after to pick up Reynif's last known survivors. Reynif was ultimately overrun, and subsequently glassed.
Team Dao was aboard Kano Station during the Battle of Earth. The Spartans defended the station from boarding Covenant infantry. While treating a wounded marine, Seung-ah was shot thrice in the right side of her back by a Covenant carbine. Although she did not flinch from her wounds due to the effects of the 009762-OO drug, Henry ordered Jim to escort her to the ship's cryo facilities, knowing that she had little time. When Kano Station took extensive damage, her cryo chamber was ejected along with others and landed on Voi.
During the same time as the Raid on Kilimanjaro, her cryo chamber was recovered by UNSC forces. Her right lung was replaced with a newly flash-cloned one. Because of her condition, Seung-ah could not participate in the Battle of Installation 00. She was brought to tears when she was told that the rest of Team Dao went missing in action on Kano Station. Soon, however, Henry-G066, Jim-G067, and Jack-G068 were recovered from Africa, having escaped Kano Station in an escape pod moments before the station's destruction.
―Seung-ah-G218, teasing Dom-094, upon seeing him without armor for the first time
The Human-Covenant War left many civilians homeless. Paradiso was one of the newly-discovered habitable planets suitable for refugees. In the year 2556, Seung-ah-G218, along with Henry-G066 and Jim-G067, was sent to the planet with a set of refugees to ensure their safety from potential insurrectionist or Covenant attacks. They arrived on November 20, where they met up with Dom-094, who was already on Paradiso to watch over the first set of refugees.
Jim-G067, Seung-ah-G218, and Henry-G066 fight off Covenant forces
Just three days after the SPARTAN-IIIs' arrival, a small fleet of Covenant Loyalists led by brutes attacked the colony. The combination of the SPARTANs and a greater number of UNSC Army and Marine ground forces annihilated the Covenant ground forces. However, the brutes were able to successfully raid a camp that was not well-guarded, kidnapping none other than Seung-ah's very own father – Gunnery Sergeant Yoon, his surviving men, and the civilians at the camp. The battle in space was also in the UNSC's favor, thanks to the UNSC's greater number of vessels compared to that of the Covenant, forcing the latter's remaining ship, a single battlecruiser, to retreat.
Seung-ah-G218 greets Dom-094, when he meets up with the SPARTAN-IIIs after battle.
―Seung-ah-G218 to Jim-G067, as the latter continues to mutilate a brute's corpse
The SPARTANs arrive on the surface of Beimeni. From left to right: Jim-G067, Henry-G066, Dom-094, Seung-ah-G218
Upon arriving at the planet the Covenant escaped to, Surkov's fleet faced another battlecruiser, with the escaping battlecruiser nowhere to be found. Shortly after the UNSC's arrival, the missing battlecruiser emerged from the planet's atmosphere, indicating that it had likely deployed its prisoners at the planet's surface. Surkov ordered the SPARTANs and a division of infantry to search for and rescue the captured humans.
The Frigg's AI, Nótt, located the IFFtransponders of the captured Marines, and the UNSC ground forces subsequently rushed to the location, which was a Forerunner tower. With the Spartans, led by Dom-094, spearheading the operation, the UNSC forces made their way to the prisoners, killing any enemy that stood in their way. Just before the SPARTANs could reach the prisoners, they arrived at the tower's control room, where Tisamenus, the chieftain leading the Covenant forces, waited for them. The chieftain mocked the Spartans by telling them he had already killed one of their kind, even pointing to her armor he had on display in the room. Dom-094, immediately recognizing the armor belonging to Annie-003, nonchalantly ordered Henry-G066 and Jim-G067 to unload theirweapons on Tisamenus. Dom-094 and the two other male SPARTANs, equipped with heavysupportweapons, obliterated the chieftain long before he could reach them with his hammer. Among the chieftain's charred remains, Jim-G067 recovered his weapon, a Tartarus' Gavel. With their biggest threat eliminated, the Spartans proceeded to free the prisoners. Just before leaving the control room, Dom-094 activated the fail-safe detonation system of Annie-003's armor, demolishing pursuing Covenant forces and honoring his deceased former leader.
The UNSC forces escorted the civilians to the nearest extraction with Jim-G067 in front, plowing through brutes and jackals with his new weapon. Once all UNSC forces evacuated the planet, Surkov ordered the orbital bombardment of the Covenant bases on the surface.
Seung-ah-G218 is best known for her very friendly and sociable personality which often boosts the morale of her allies. Due to her relative lack of combat ability among the Gamma Company SPARTANs, Seung-ah initially had difficulty finding her place among her peers. Her communication skills, however, caught the attention of Aspen-B145, who took it upon himself to train her as a medic. The older SPARTAN's decision proved to have paid off; Seung-ah had since gained a reputation of keeping almost all wounded allies alive long enough to receive proper medical attention, regardless of how terrible the injuries are. Those who have been treated by her describe her as "an angelic, reassuring presence" or "giving off a soothing aura." Although Seung-ah is inferior in combat when compared to her fellow Spartans, she herself is still a Spartan and is therefore a skilled soldier in her own right, outperforming most of the other soldiers of the UNSCArmed Forces on the battlefield.
Because of Seung-ah-G218’s outgoing and friendly character, she inevitably got along with almost every other SPARTAN-III of the Gamma Company. Inevitably, with her time spent with Team Dao, she became very close friends with all of her teammates. Seung-ah considered April-G182 to be her best girl friend and was saddened when the latter was withdrawn from the team, knowing that she will likely never see her again. Seung-ah also looks up to Dom-094 and often tries to engage in conversations with him in order to learn more about the Spartan who saved her from the Covenant.
Seung-ah-G218's physical appearance is modeled after that of the singer and actress Yoona.
Seung-ah coincidentally shares her name with the actress Yoon Seung-ah, as Hyper Zergling had no knowledge of the actress upon the creation of this character.
↑Many elements of Aspen-B145's biography are not consistent with, and he therefore cannot exist in, the Platinum Annals Universe. While it is Aspen who trains Seung-ah in The Onyx Chronicles, Seung-ah, instead, receives her additional medical training through unknown means in the Platinum Annals Universe.