Rookie, identified as CT-3899, functioned as a clone trooper within the ranks of the Grand Army of the Republic.
Rookie's initial deployment involved being assigned to sergeant Kano's unit on Ryloth. This occurred following the liberation of Lessu, the planet's capital city. Under the leadership of Jedi General Anakin Skywalker and Captain Rex, Kano, Rookie, and other newly trained troopers received the mission to eliminate the remaining Separatist forces from the city of Resdin. Rookie displayed a strong desire to prove his capabilities and demonstrate his worth to his comrades. After the final group of droids was eliminated, Kano acknowledged the "shinies" for successfully completing their initial task, observing that they had shed their "shiny" status.