RK-9, alternatively 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 participated in a battle during the Clone Wars on Mon Cala. It was reconstructed by the time that the war between the First Order and the Resistance was occurring. Following an attack by Quarren in Dac City, Gee brought the Resistance members Rey, Rose Tico, and Chewbacca before a court. He then proposed resolving the dispute through an ancient trial by combat, which Rey accepted, anticipating a duel with Gee himself. However, Gee instead presented RK-9 as his champion for the trial, which took place in the Sacred Arena of Salka'a. During the ensuing fight, RK-9 gained the upper hand against Rey, but its actions endangered several onlookers, prompting Tico to intervene and destroy RK-9.
During the Clone Wars in 20 BBY, RK-9 was among the numerous aqua droids deployed by the droid army of the Confederacy of Independent Systems during the Battle of Mon Cala. This battle was fought against the Galactic Republic on the planet Mon Cala. RK-9 met its initial end during this conflict, but it was later rebuilt and assigned as the personal security droid to Chadkol Gee, who served as the advisor to Quarren General Nossor Ri.
In 34 ABY, some Resistance members journeyed to Mon Cala to request assistance in their fight against the First Order. However, after an altercation involving a fight with some Quarren attackers, Resistance members Rey, Rose Tico, and Chewbacca found themselves on trial before King Ech-Char in the Mon Cala Court Of Justice. Gee proposed resolving the matter through the ancient rite of challenge, a fight to the death. Rey accepted, believing Gee intended to fight her personally, but the Quarren instead introduced RK-9 as his champion for the combat.
The rite of challenge took place in the Sacred Arena of Salka'a, where RK-9 and Rey faced each other before a crowd of onlookers. Rey successfully evaded RK-9's attacks and managed to remove a section of the droid's armor, revealing its processor. However, RK-9 struck Rey with its arm and fired its laser cannon at her after she had fallen. The shot missed, landing dangerously close to several spectators. The droid fired another shot, nearly hitting a member of the Mon Calamari Guard. Rey attempted to divert RK-9's attention, urging the bystanders to evacuate. As RK-9 prepared to fire at the downed Rey, Tico intervened, using a blaster spear taken from a Mon Calamari Guard to sever the aqua droid's head. Although Rey survived the encounter, Tico's intervention in the challenge resulted in the Resistance members being exiled.
RK-9 was an aqua droid, a sophisticated form of battle droid created by the Techno Union for use 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 function as a circular spinning blade. As an aqua droid, RK-9 had a height of 2.83 meters.
Arkay-Nine made an appearance in Allegiance 3, a comic written by Ethan Sacks, with illustrations by Luke Ross. It was released by Marvel Comics on October 23, 2019.