The Knights of the Round (ナイトオブラウンズ, Nait ō bu Raunzu) or formerly known as the 12 Horsemen is a unit of twelve elite soldiers in the Holy Empire of Britannia, each being assigned into the group by royalty and under direct command of the Emperor. The Knights of the Round extends to at least the 18th century, with the first Knight of One being Sir Richard Hector. Named after the Arthurian Knights of the Round Table, they operate outside the standard Britannian military and are given special privileges and authority.

http://codegeass.wikia.com/wiki/Knights_of_the_Round#Known_Members