The Royal Guards, as its name suggests, are military formations whose duty is to protect and serve members of the Britannian royal family. They wear a variety of uniforms depending on who they are affiliated with. There appear to be a branch of Royal Guards in the Knightmare Corps, as seen during the Second Battle of Kamejima Island under the command of Monica Krushevsky. It is implied that the Knights of the Round may also be assigned Royal Guards. What happened to Charles' Royal Guards during or following Lelouch's rise to power is not explained; they may have joined the uprising, or they may have been absorbed into Lelouch's Royal Guards under the influence of his Geass.
The Imperial Guards are mainly seen in the Imperial Palace, serving the Emperor directly. They wore gray uniforms with red sashes and short black and red capes, and carried rifles with elaborate bayonets, possibly more for ceremonial purposes than for actual combat; (the Swiss Guards of our world are the personal bodyguard of the Roman Catholic Pope, and similarly carry polearms). They are connected in some fashion to the OSI, evidenced by a mention of them in one of Rolo's case files (Counterattack at the Gallows) and that they wear the same helmets. Jeremiah Gottwald was a member of the Imperial Guard during his time at Aries Villa. Cornelia wore their uniform (with her trademark cape) while commanding Marianne's guards. Gino complains that the Imperial Guards rely on the Rounds for everything, and then complain the Rounds are robbing their salary. Two members were seen deployed at the Special Administrative Zone of Japan's opening ceremony, where they blocked Zero from coming on stage after he Geassed Euphemia. He attempted to Geass them to "stand aside," but all three were distracted by Euphemia shooting General Darlton. Their fates are unknown once the massacre began.
Charles' Royal Guards are also seen later piloting green and gold Vincents.
Clovis' Royal Guards accompanied him to the Shinjuku Ghetto in Area 11 to try to recover C.C. after the capsule containing her was stolen by the Japanese resistance. They were the first Royal Guards to be seen in the series, and were among the only people who knew what the capsule they were sent to recover actually contained; lower-ranking soldiers and civilians were led to believe that it contained poison gas. They are only shown wearing their field uniforms which were dark red versions of the standard Britannian Army uniforms while officers wear flat caps and a black baldric or short shoulder cape. A squad of them became the first people to be affected by Lelouch's Geass and were ordered to kill themselves by him as his first step towards defeating Britannia.
A Royal Guard officer
Clovis' Royal Guard Preparing to Fire
Clovis' Royal Guards Under the Influence of Lelouch's Geass.
Schneizel's Royal Guards, who were led by Kanon Maldini, protected him and carried out his orders. They wore green uniforms, and occasionally wore white capes. They are generally seen accompanying Kanon or Schneizel in the background. They were shown to wield ceremonial rifle polearms as the Imperial Guards do.
Schneizel's RoyalGuards flanking him
Kanon in Schneizel's uniform
Schneizel,Cornelia,DiteHard Nd Nunnally with Schneizel's RG's at damocles.
Cornelia's men are the most commonly seen Royal Guards in the series who employed them very often alongside her throughout the first season along with the Glaston Knights. Her Royal Guards wore maroon-red uniforms and short shakos. They piloted Gloucesters and were extremely competent in combat. In the climatic Assault on Tokyo Settlement, led by the Glaston Knights they were instrumental in defeating the Black Knights and ending the first rebellion. They are still seen alongside Guilford from time to time in the second season.
Cornelia give orders to her Royal Guards.
Cornelia's Royal Guard officers
Guildford with some members of the royal guard. Notice the blue facings here which are similar to the Glaston Knights
Following his rise to power near the end of the series, Lelouch used his Geass to force the service of many Royal Guards for himself, forcing them to swear absolute loyalty to him to the point of sacrificing their lives to ensure his own survival. All members are dressed in white-and-gray uniforms vaguely similar to the Knights of the Round with black tricorn hats and grey half masks, the jacket and hat emblazoned in goldeneye symbols similar to the ones on Lelouch's Imperial outfit. The tricorns also have drop-down visors similar to the one on Zero's helmet. They became the face of the terror unleashed during his reign.
The Knights of the Round are known to have had their own personal forces. Little is known about their operation or uniforms, but they were ultimately defeated by Suzaku when they tried to attack Lelouch near the end of the series during the Knights of the Round Uprising. Whether these personal forces were legitimate Royal Guards is not explicitly stated.