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.
Euphemia is introduced in the fifth episode, where she jumps from her window several stories above the ground and lands in Suzaku Kururugi's arms. They become fast friends and Euphemia, after revealing her identity as the third princess of Britannia, asks that he help her prevent any unnecessary deaths.
When she is taken hostage along with a group of tourists by the Japan Liberation Front at the Lake Kawaguchi Convention Center Hotel, Euphemia attempts to use her position to free the other hostages. Lelouch, in his Zero persona, has incidentally led the Black Knights to the same location, and the two meet. Lelouch remarks that Euphemia had not changed since childhood and was not surprised to see her act in such a manner. Though he brandishes a gun at her, he decides against killing her.
Concerned about Euphemia's safety, Cornelia persuades her younger sister to select a Knight to protect her. After resolving the conflict between her own ideals and Cornelia's advice, Euphemia chooses Suzaku Kururugi. Cornelia worries that Euphemia's ideals will get her sister into trouble and warns Euphemia that the policy of discrimination against Honorary Britannians like Suzaku are so firmly established that she might have to be exposed to the rigors and dangers of competing for the throne in order to challenge it. Regardless, Euphemia adamantly sticks to her decision and Suzaku remains her Knight until he voluntarily steps down after he is arrested for deserting the frontlines during a battle against Zero. Euphemia convinces him to rejoin her, eventually declaring her love for him.
In the immediate aftermath of the Battle at Shikine Island Base, Euphemia encounters Zero when the two of them are stranded on a nearby island. Having deduced Lelouch's identity from their earlier meeting, she convinces him to unmask, promising to keep his alter ego a secret. The incident allows her to reconnect with Lelouch and Nunnally as she secretly visits them at the Ashford Academy during the school festival. When the media catches her visiting the school, Euphemia makes a surprising announcement. Wishing to live the peaceful days she once had with Nunnally and Lelouch, she declares the establishment of the Special Administrative Zone of Japan for the Japanese people. This runs completely contrary to Lelouch's plans. Though met with disapproval from other government officials, her plans go forth with the help of her older brother Schneizel El Britannia, who apparently supports her idea and promises to inform Cornelia of it. Lelouch, when he meets her as Zero, reveals 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 confides in her 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. Euphemia initially tries to resist the order and briefly succeeds, but is soon overtaken by its power. Under its influence, she leads the Britannian army to massacre countless innocent people. Lelouch, with much regret, fatally shoots her to end her rampage while revealing that he may have considered Euphemia his first love. He nevertheless takes advantage of the chaos, rallying the angry Japanese populace into a massive army and leading it against the Britannian forces.
Euphemia's body is recovered by Suzaku and she spends her final moments with him. The command to kill him, as he is of Japanese descent, tries to take hold, but she resists it, either because she cares for him too much or is simply incapable of carrying it out. As she is unaware of what she did, Suzaku tells her that her proposal was a success, allowing her to die believing that the Japanese people were happy with the Japan she had created. Her death creates a firm resolve in both Suzaku and Cornelia to defeat Zero once and for all — precisely the situation Euphemia had wished to avoid.
In supplementary materials, it is revealed that Britannia officially declared Euphemia responsible for the massacre and that she had been deprived of royal status and executed by the Britannian Army. After the incident, the Japanese general public in Area 11 refer to her as the "Massacre Princess" (虐殺皇女|Gyakusatsu Koujo,"Princess Massacre" in the English dub).
Euphemia's death impacts and influences the actions of several characters. Suzaku becomes emotionally unstable and pursues Zero, having discovered Zero's true identity. Once he believes Zero has been eliminated, Suzaku continues to serve the Britannian Empire. His actions remain motivated by his memory of Euphemia; he refers to her as an "irreplaceable woman" when he speaks to Lelouch on the rooftop of Ashford Academy and carries around Euphemia's quill pen as a memento.
Euphemia's half-sister Nunnally, with her installment as the new Viceroy of Area 11, chooses to continue Euphemia's ideal and reinstate the Special Administrative Zone of Japan. As she reveals this to Zero, unaware that he is Lelouch, he is visibly unnerved by how Nunnally's offer reminds him of Euphemia. In order to spare Nunnally the same fate as Euphemia, Lelouch has himself and the entire Order of the Black Knights exiled.
Once Lelouch becomes Emperor, he explains to Suzaku that he intends to shed enough blood to make the name of the Massacre Princess be forgotten. In this aspect, Lelouch succeeded; following the Zero Requiem, all hatred was focused on Lelouch and the past forgotten, including Euphemia's massacre.