Euphemia li Britannia (ユーフェミア・リ・ブリタニア, Yūfemia Ri Buritania?), 16 years old, is the third princess of the Britannia Imperial Family, Euphemia is Lelouch Lamperouge's half-sister and the sister he cares most about after Nunnally. Euphemia is the Sub-Viceroy of Area 11, a role she admittedly does not enjoy. Euphemia admires her older sister, Cornelia, but despises all combat in general, though is capable of piloting a Knightmare Frame, as seen in the battle at the Empire's Shikine Island Base.

Though she is naive and innocent to a fault, with her ideals frequently conflict with reality, she remains kindhearted and open-minded to everyone around her. She is the second person in the Imperial Family to discover that Lelouch is alive and first to know his alter ego as Zero, having deduced it after the hotel hijacking incident. In childhood, she was quite close to Lelouch and Nunnally. She develops a close, romantic bond with Suzaku Kururugi because of their shared belief that a problem must be changed from within.