The Adeptus Custodes are the personal bodyguards of the Emperor. They are roughly similar to the Space Marines in that they are superhumans grown from a gene-seed. Unlike the space marines, their gene-seed is not from any primarch, but is instead custom-tailored by the Emperor himself. They are taller, stronger, and hardier than space marines, and use specialized wargear. They maintain a rigorous training program known as the Blood Games, where selected Custodes attempt to break into the Imperial Palace, while the other Custodes must try to stop them.
After the fall of the Emperor, the Custodes have never left the Imperial Palace. They wear black as a sign to show their mourning, and only allow a select few to have an audience with the Emperor. They remain largely aloof of the state of the Imperium, save for times when Terra is directly threatened.
With the exception of Kitten, all Custodes have become insane and have stripped off their armor. A few are based on old first edition Warhammer art, but most are based on various characters from JoJo's Bizarre Adventure, particularly the Fabulous Custodes.