Euphemia li Britannia is a major character in the first season of the anime series Code Geass. She is the Third Princess of the Holy Britannian Empire and half-sister of the series' protagonist, Lelouch vi Britannia. She is voiced by Omi Minami in the original Japanese version and Michelle Ruff in the English dub.
Euphemia - often called by her nickname "Euphie" - makes her first appearance in the series fifth episode, The Princess and The Witch. She possesses a very kind and innocent personality and unlike other members of Britannian royalty, she does not look down on non-Britannians and finds referring to people as "numbers" distasteful. During her childhood, Euphie was very close with her half-brother Lelouch and his little sister Nunally. After the death of her brother Clovis, she and her sister Cornelia travelled to Area 11 where Cornelia took up Clovis' former position as viceroy with Euphie becoming sub-viceroy. The first person she meets after arriving in Japan is Suzaku Kururugi, an honorary Britannian and a pilot in the Special Corps. The two meet after Euphie boldly jumps out of a window and into Suzaku's arms.
Shortly after taking up her position as sub-viceroy, Euphemia becomes entangled in the Japan Liberation Front's hijacking of the Kawaguchi Convention Center Hotel, becoming one of the many hostages. Her identity remains unknown to the JLF members until one of them threatens a Britannian student, Nina Einstein. Euphie then announces her identity and her captors take her to their leader, who by then is negotiating with the Black Knights' leader Zero. Her revelation causes the negotiations to break down and force Zero to use Geass to kill Kusakabe and his men. Shortly after meeting Zero, Euphemia deduces his true identity as her half-brother Lelouch, long ago exiled from the Britannian homeland. She keeps this knowledge to herself throughout the series, only revealing to Lelouch himself that she knows by Episode 19, Island Of The Gods.
Euphemia shares Suzaku's ideal of changing the system of government from within and eventually works toward establishing peace in Area 11 with the intention of founding an autonomous zone where the Japanese can live freely without prejudice from the Britannians. When she announces her plan to establish this administrative zone, Lelouch is outraged, believing such a plan conflicts with his own goals.
In the 22nd episode Bloodstained Euphie, Lelouch and Euphemia meet face to face just as the opening ceremony of the Special Administrative Zone is about to begin. Lelouch discusses his opposition, but becomes convinced of her conviction when she reveals she is doing it for the sake of Nunnally and has abdicated her right to succeed the Britannian throne. Upon agreeing to Euphemia's proposal, Lelouch goes off on another conversation with Euphemia, eventually saying that if he really wanted to, he could get her to do anything he asked for, and he mentions genocide of the Japanese people as an extreme example. Unknown to him, minutes ago he had lost his ability to turn his Geass off, and it conveys the order to Euphemia. Seized by the dominating power of Geass, Euphemia takes the stage at the ceremony and politely announces her request to the Japanese people "Could you please all die?" When the flabbergasted audience don't immediately start killing themselves, Euphie orders the Britannian troops to open fire on the crowds, leading to countless innocent deaths and the total collapse of Euphemia's plans for peace. As the carnage erupts, the brainwashed Euphemia takes an assault rifle and starts gunning down every Eleven in sight. She even takes a Sutherland Knightmare Frame out into the city to continue the slaughter until she is found by Lelouch, who disables her robot and then confronts her face to face. Unable to rescind his accidental Geass command, Lelouch sorrowfully pulls a gun and shoots Euphemia. After Lelouch flees the scene, Suzaku recovers Euphemia and takes her to the Avalon aerial carrier. The wound is fatal and nothing can be done for her, but in her last moments Euphie asks Suzaku how everything turned out, Geass' influence fading. Suzaku reassures her that everything went fine and Euphemia passes believing that the Japanese were free and happy.
It is important to note that when Lelouch accidentally used Geass on Euphemia, she seemed to resist the command to kill all the Japanese. While it could have been due to Lelouch's loss of control over his power, it seems more likely that such a malicious and barbaric order conflicted too much with Euphemia's selfless and gentle nature.