During the era of the Clone Wars, which is considered a "Legends" storyline, Clone Sergeants were clone troopers specifically created on Kamino. Their purpose was to lead squads within the Grand Army of the Republic. Additionally, they oversaw the training of clone cadets belonging to the Clone Youth Brigade.
Clone trooper sergeants typically wore Phase I clone trooper armor distinguished by olive green markings on both their helmets and torso during the early stages of the Clone Wars. However, some clone trooper sergeants did not display these green flashes on their armor. Examples include the sergeant who assisted Anakin Skywalker's pursuit of Asajj Ventress on Yavin 4 following the Battle of Muunilinst, as well as two sergeants without markings who participated in both the Battle of the Rishi moon and the Battle of Ryloth.
Clone sergeants held a non-commissioned officer rank within the Grand Army, placing them directly in combat alongside the troops they commanded. Each sergeant was responsible for leading a squad consisting of nine clone troopers. Clone lieutenants then coordinated these squads, organized under the sergeants, into larger platoons.
The sergeant rank was also assigned to ARC troopers, though their specific command roles are not fully known. These ARC sergeants also had green markings. The most well-known of these sergeants was Null-class Advanced Recon Commando Null-12, who was nicknamed A'den. Another ARC sergeant worked alongside Anakin Skywalker during the Battle of Praesitlyn, during an operation to rescue hostages.