Julius Kingsley is a Britannian military advisor deployed to St. Petersburg in order to take charge of the operational planning for the war effort being carried out by the forces of Euro Britannia against the E.U. He was escorted by Suzaku Kururugi during his travel to Europe on a special train.

In episode 3 of Akito the Exiled, Julius was revealed to indeed be a brainwashed Lelouch who is completely loyal to the Emperor of Britannia. His eye-patch hides the Geass. There are multiple scenes throughout the OVA series that suggest his state is relatively unstable, with his original personality briefly resurfacing, as indicated by Kinsgley muttering Nunnally's name.
(Source: Code Geass Wikia)