Cornelia li Britannia (コーネリア・リ・ブリタニア, Kōneria ri Buritania) is the Second Princess of the Holy Britannian Empire and Commander-in-Chief of the Imperial Army. Extremely skilled in Knightmare Frame combat, she establishes Britannian rule in the Middle Eastern Area 18 shortly before her appointment as Viceroy of Area 11 following Clovis's death. She has a strong distrust of foreigners, even those with Honorary Britannian status, and prefers to win without their help. Though she is decisive and straightforward in most situations, she has a soft spot for her sister Euphemia, such that she prioritizes Euphemia's safety over the completion of her objectives. Her personal Knight is Gilbert G.P. Guilford, who commands the Glaston Knights.

Upon arriving in Area 11, Cornelia launches a military campaign against Zero's Black Knights and the other rebel factions, crushing all but the Black Knights. After Euphemia's death, Cornelia confronts Zero and nearly kills him; however, she is impeded by her subordinate Andreas Darlton, who is under the influence of Lelouch's Geass. Darlton injures her and disables her Knightmare Frame. She then learns that Zero is Lelouch and, through the use of Geass, answers his questions regarding the murder of his mother. Later, when Suzaku finds her, she knights him and sends him after Lelouch. In the aftermath of the Rebellion, Cornelia is labeled MIA by Britannia, as she goes underground to expose the existence of Geass in order to clear Euphemia's name. However, upon infiltrating the Geass Order and learning of her father's ties to it, Cornelia is captured by V.V. until the Black Knights' assault on the Order allow her to break out of her cell. She aids Lelouch in taking down V.V.'s Siegfried using a damaged Akatsuki (rigged with numerous salvaged weapons) to attack its blind spot, allowing her and Lelouch to deliver the finishing blow in tandem. Following this, she is taken prisoner by Lelouch, but manages to escape during the second battle of Tokyo aftermath, joining Schneizel when he arrives as an envoy and telling him everything she learned relating to the Geass Order.

She joins his faction in Cambodia after Lelouch becomes Emperor. When she learns of Schneizel's plan to position Damocles in such a way that it could strike all over the world at the same time, she tries to stop him, only to be gunned down. She is taken from the Damocles to Horai Island, where she is treated for her wounds and reunited with Guilford. Two months later, after the culmination of Lelouch's plans (the Zero Requiem), she is shown leading a small resistance group (including Villetta and Guilford). They take advantage of Lelouch's assassination to restore order. She is last seen in a photo for Villetta's wedding with Guilford by her side.

She is not present in the manga adaption, with the position of Viceroy going to her sister, Euphemia.