RK-9, also known as Arkay-Nine, was a re-engineered aqua droid functioning as the dedicated security droid for Chadkol Gee, an advisor serving General Nossor Ri on the planet of Mon Cala. This droid had previously engaged in combat during the battle of Mon Cala in the Clone Wars, and was later restored by the time of the war between the First Order and the Resistance. Following an assault by Quarren in Dac City, Gee brought Resistance members Rey, Rose Tico, and Chewbacca before the court. He then proposed resolving the dispute through the ancient rite of challenge, a proposition accepted by Rey, who assumed she would be battling Gee himself. However, Gee instead nominated RK-9 to represent him in the challenge, which took place in the Sacred Arena of Salka'a. During the confrontation, RK-9 gained the upper hand against Rey but endangered onlookers, leading Tico to intervene and ultimately destroy RK-9.
During the Clone Wars in 20 BBY, RK-9 was among the numerous aqua droids utilized by the droid army of the Confederacy of Independent Systems in the Battle of Mon Cala. This battle was fought on the planet Mon Cala against the forces of the Galactic Republic. Although RK-9 was initially destroyed in this conflict, it was subsequently rebuilt and went on to serve as the personal security droid of Chadkol Gee, who was the advisor to the Quarren General Nossor Ri.
In 34 ABY, some Resistance fighters journeyed to Mon Cala seeking aid in their fight against the First Order. Following an altercation involving a fight with some Quarren assailants, Resistance members Rey, Rose Tico, and Chewbacca were brought before the Mon Cala Court Of Justice, presided over by King Ech-Char. Gee proposed that the matter be resolved through the ancient rite of challenge, a fight to the death. Rey agreed, expecting to face her in combat, but the Quarren instead presented RK-9 as his champion for the duel.
The challenge took place in the Sacred Arena of Salka'a, with RK-9 and Rey confronting each other before a crowd of spectators. Rey successfully evaded RK-9's initial attacks and managed to dislodge a section of the droid's armor, revealing its processor. However, RK-9 retaliated by striking Rey with its arm and firing its laser cannon at her after she fell to the ground. The shot missed, narrowly avoiding several bystanders. The droid then discharged another blast, nearly hitting a member of the Mon Calamari Guard. Rey attempted to create a distraction, urging the onlookers to evacuate. As RK-9 prepared to fire upon the downed Rey, Tico intervened, using a blaster spear she had taken from a Mon Calamari Guard to sever the aqua droid's head. Although Rey survived, Tico's intervention in the challenge resulted in the Resistance members being exiled.
RK-9 was an aqua droid, a sophisticated type of battle droid produced by the Techno Union for deployment on aquatic worlds. Following its reconstruction after the Battle of Mon Cala, RK-9 functioned as a security droid. RK-9 featured a red photoreceptor and gray plating, which had become corroded by the time of its destruction. The aqua droid was armed with a retractable laser cannon on its right arm, which could also be used as a circular rotating blade. Being an aqua droid, RK-9 had a height of 2.83 meters.
Arkay-Nine made an appearance in the comic Allegiance 3, which was penned by Ethan Sacks, illustrated by Luke Ross, and released by Marvel Comics on October 23, 2019.