Atin (pronounced /aah-TEEN/), which in Mando'a translates to "persistent" or "stubborn," became the chosen moniker of a clone commando serving with the Galactic Republic's Omega Squad during the Clone Wars. He was designated RC-3222 and received his training from Sergeant Walon Vau.
Throughout his training period, Atin accumulated several battle scars. These injuries resulted partly from the rigorous training regime, but also from the actions of Walon Vau. Vau, a notoriously harsh training sergeant, frequently forced his soldiers to fight each other or even himself. After Atin's first squad perished during a training exercise attempting to rescue him, Vau compelled Atin to make a choice. Unwilling to fight his clone brothers, Atin chose to confront Vau instead. Vau nearly killed Atin using a Mandalorian saber. Kal Skirata considered Vau's brutal training methods sadistic. Following his survival as the sole member of his initial squad, he was assigned to a second squad.
In the year 22 BBY, Atin experienced the loss of his squadmates during the Battle of Geonosis, which led to his reassignment. To help Atin overcome his survivor's guilt following the deaths of his squadmates, Vau inflicted a scar upon the clone with his knife. This scar stretched from Atin's right eye to the left side of his chin, making him easily distinguishable from other clones. Soon after, Atin became a member of the newly established Omega Squad, alongside three other clones who had also lost their respective squads: Darman, Fi, and Niner.
Three months after the Battle of Geonosis, Atin joined Omega Squad on a mission to the planet Qiilura. He played a role in capturing the Separatist scientist, Doctor Ovolot Qail Uthan, and in defeating the Separatist forces stationed there, which were commanded by Ghez Hokan. During this mission, he sustained an injury inflicted by Hurati.
One month later, Atin took part in the Galactic City spaceport hostage siege, contributing to the elimination of the Korunnai terrorists.
During a mission at the Corellian–Perlemian hyperspace intersection, Atin and the rest of Omega Squad apprehended terrorists linked to a terror cell operating on Coruscant. Alongside Delta Squad and Kal Skirata, they were tasked with dismantling the terrorist network. This mission led to Atin's reunion with his former training sergeant, Walon Vau. Sev, a member of Delta Squad who had also trained under Vau, initially felt conflicted towards Atin and attempted to shield Vau from him, but Kal Skirata facilitated their cooperation. Following the mission's conclusion, Atin seized the opportunity to confront Vau. He engaged Vau in combat wearing only a black bodysuit and armed with a standard-issue commando vibroblade. Their fight continued until Atin gained a position to deliver a fatal blow to Vau, but Bardan Jusik intervened, using the Force to separate them and ordering Atin to stand down.
During the Coruscant mission, Atin developed a romantic relationship with a Twi'lek waitress named Laseema.
Approximately 65 days after the Coruscant mission, Atin accompanied Prudii, a Null-class ARC trooper, on a mission targeting the droid factory located on Olanet. Atin volunteered for this assignment, while the rest of Omega Squad rested on Coruscant, to learn Prudii's techniques for infiltrating droid factories and sabotaging them to produce substandard droids that could be more easily defeated.
The mission was a success, but during their time there, Atin began to question the official Republic Intelligence estimates regarding the strength of the Separatist Droid Army. While they claimed the existence of quadrillions of droids, Atin only observed evidence suggesting a few hundred million on Olanet.
They were extracted from Olanet by a TIV, which, under Prudii's direction, diverted course to attack a Separatist droid transport near Cato Neimoidia. Atin initially considered this action reckless, given the possibility of the droids being part of a spiked batch, but Prudii explained that ceasing to engage Separatists at every opportunity could alert them to the Republic's sabotage efforts. Subsequently, they proceeded to a rendezvous point near Drall in the Corellian system to meet with Kal Skirata, who had recently returned from a mission on Kamino. Atin was happy to see him again.
One week later, Atin joined Omega Squad on a mission to Gaftikar to assist A'den, a Null-class ARC trooper, in training the local Gaftikari resistance forces. They were transported aboard the Core Conveyor, along with the 2nd Airborne Company of the 212th Attack Battalion. The clones of the Airborne Company displayed animosity toward Omega Squad due to their Mandalorian heritage. Atin inquired about the source of their resentment and discovered that the Airborne Company had suffered significant casualties fighting against the Mandalorian Protectors. Atin found this incident troubling, marking his first encounter with clone troopers disliking other clone troopers, despite his awareness of the rivalry between the Nulls and the Alpha-class ARC troopers. Afterward, he sought to distract himself by reading a letter from Laseema.
Atin and the rest of Omega Squad were airdropped onto Gaftikar, utilizing Clone paratrooper wings attached to their armor, and were soon located by A'den, who guided them to the rebel camp. The responsibility of training the local Marit resistance fighters had initially been assigned to Sull, but he had gone MIA. The Marits established positive relationships with all the clones, including Atin. When offered some of their stew, A'den advised him "not to ask" about its contents, and he readily accepted. However, he later discovered that the Marits practiced cannibalism, and the stew contained the grandmother of one of the lizards, which caused Atin to vomit. Fi made jokes about this.
Subsequently, he accompanied Darman on a reconnaissance mission into Gaftikar's capital city, Eyat, where they encountered Sull. They tracked him to his apartment, and a confrontation ensued, during which Sull attacked them and struck Atin in the groin with his knee. However, Atin managed to hold a vibroblade to the ARC's throat. They returned him to the camp and handed him over to A'den. Sull was convinced that the Republic intended to kill him, a suspicion that was confirmed when two Covert Ops clone troopers attacked Darman, mistaking him for Sull. Darman killed the troopers, and Atin dug a grave for them, providing them with a Mandalorian-style funeral.
Several days later, he participated in the Battle of Gaftikar with the rest of Omega Squad, tasked with identifying comm stations on the planet for V-19 Torrent starfighters to destroy. Afterward, they moved to eliminate the Gaftikar HoloNet relay center. Atin ascended to the roof to disable the satellite relay up-link, but the building suddenly exploded, resulting in a broken ankle. Fi fared worse and fell into a coma, so Atin maintained watch by his bacta tank aboard the Leveler, because he had bad memories from being in bacta, so he wanted to be there to comfort Fi in case he came around.
Atin was distinguished by a prominent scar on his face, which was still healing during Omega's Mission to Qiilura. He possessed a moderate sense of humor, often responding to Fi's jokes. However, he became extremely serious during combat, a trait shared by all Clone Commandos.
As the team's computer and technology specialist, Atin had extensive knowledge of communication and technology, as well as a basic understanding of demolitions, although Darman, Omega Squad's expert in that field, handled most of those operations. Atin discovered the Katarn Armor's vulnerability to Verpine Shatter Guns through firsthand experience, resulting in considerable pain, although it did not cause serious injury.