Euphemia li Britannia

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 Britannian Imperial Family, Euphemia is Lelouch vi Britannia'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.

Personality
Though she is a member of the Britannian royal family, she remains kind-hearted and open-minded to everyone around her and treats everyone with respect even if they are not Britannian. 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 her 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.

First Season
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 spend the day in Area 11 and Euphemia goes under the name Euphie, which becomes the nickname that Suzaku and Imperial Family call her by. They become friends quickly and Euphemia, after revealing her identity as the third princess of Britannia, asks that he help her prevent any unnecessary deaths.

She attends Prince Clovis' funeral and hears the Emperor of Britannia, her father, make a speech on the virtues of inequality and the social battle for evolution and betterment.

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 rigours 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 front lines during a battle against Zero. Euphemia goes into a state of depression as she feels she is useless and the blow of Suzaku leaving her but an encounter with Nina allows her discover that she has developed feelings for Suzaku. Suzaku at that moment goes into battle with a new prototype that is connected to Lancelot. His energy levels drop and Euphemia makes a transmission and declares that she orders him to love her and in return she will love every bit of him. Suzaku reciprocates the feelings and declares his love for her. Euphemia then convinces him to rejoin her as her knight.

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 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.

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. Trying to hide the unforgettable crime he made, Lelouch made all Elevens and the Order of the Black Knights believe that Euphemia betrayed them, that she just thought of the Administrative Zone of Japan a laugh and made fun of all Japanese. Lelouch then ordered to stop Euphemia from the massacre of all Japanese and quoted: "Stop the Britannians! Save the Japanese!" He suddenly become teary-eyed as he ordered, "...and kill Euphemia!" Lelouch, with much regret, fatally shoots her to end her rampage while revealing that he 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 and asks if she made the right decision, Suzaku decides to lie and tells her that the Japanese were happy with the Japan she created. She cries with joy telling him that she is glad it worked out. Suddenly she can't see his face anymore and the both of them begin to cry. Realizing she has little time Euphemia urges Suzaku to stay in school for the both of them and dies smiling telling him how glad she is. Her death creates a firm resolve in both Suzaku and Cornelia to defeat Zero once and for all.

Aftermath
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; aside from this false declaration, Britannia refused to make any other compensation for the Japanese, instead they increased the level of discrimination; to the point where the Britannian mafia were able to freely do human trafficking and other illegal activities on the Japanese. "Citizens of Japan" are reverted back to the name Elevens, and a number of initially restored rights and privileges are again denied to Elevens. After the incident, the Japanese general public in Area 11 refer to her as the "Massacre Princess".

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. Euphemia's name was ultimately cleared after The Black Knights learned about Lelouch's actions and the existence of Geass. However, Schneizel did not reveal the whole truth, implying that Lelouch had intentionally compelled her into causing the massacre.

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.

Abilities
She was able to resist geass, despite it quickly overcoming her.

Suzaku Kururugi
She confesses her feelings towards Suzaku and learns he loves her back. Suzaku was emotionally crushed when Zero killed Euphemia. Therefore, throughout both seasons, he is shown hating Lelouch, when it was revealed that he was Zero, and having the intent to kill him and make him suffer for the death of his most loved one.

Cornelia li Britannia
Euphemia is Cornelia's younger sister, and loves her sister very much, she left her throne to clean the name of her sister as the massacre princess.

Lelouch vi Britannia
Euphemia is Lelouch's younger sister, and was told by Lelouch that she was his first love shortly before her death at his hands due to him losing control of his Geass. The Island of the Gods episode also boasts strong innuendo about their relationship.

Code Geass: The Manga
After the death of Clovis, she soon takes over as the Governor General of Area 11 and meets Lelouch sooner as a temporary student in Ashford Academy. She shares the same fate as she did in the anime, where she had set up the Special Administrative Zone of Japan only to have it end in a disaster as she was accidentally Geassed by Lelouch to kill all the Japanese. Euphemia was later killed by Zero, who used her actions to stir up the Black Rebellion.

Nightmare of Nunnally
In Nightmare of Nunnally, Euphemia becomes the Governor General after Clovis' death, and picks Suzaku Kururugi as her knight, dispatching him to assist a beleaguered Princess Cornelia in a three-way battle between Nunnally's Mark Nemo. When it is discovered that Nunnally is the pilot and is to be executed on orders of Rolo vi Britannia, she recruits Suzaku and the Lancelot to disrupt the execution, where they are subsequently aided by Zero. In the ensuing Four-way battle (between Zero, Suzaku, Rolo and Alice), Nunnally is spirited away by Alice. When Emperor Charles zi Britannia abolishes the Holy Empire of Britannia for the Holy Nation of Eden Vital, Euphemia is arrested by Rolo along with Cornelia (and all other governor-generals). She and Suzaku are rescued by Zero and the Kyoto House, and she becomes the 99th Empress of Britannia due to support by Kyoto House, the (now-imprisoned) Imperial Senate and Prime Minister Schneizel, as well as her sister Cornelia. She leads the Britannian army against the Emperor at the Sword of Akasha, but her forces are routed by the Knights of Round (who in NoN are essentially undead zombies). Fortunately, they were prevented from being annihilated due to the Knights' demise as a result of Charles' death. At the end of NoN, Euphemia frees all the numbered areas, trying her best to establish peace between these newly created nations.

Nunnally in Wonderland
Euphemia plays the role of the White Queen in Code Geass: Nunnally in Wonderland. When the Queen of Hearts (Marianne vi Britannia) challenges Nunnally to a game of croquet, Euphemia appears alongside the White King (Cornelia Li Britannia) demanding to play as well, saying that she wants to make it so that croquet can be played by everyone. She even states that she wants to establish a Special Administrative Zone of Croquet.

Code Geass: Lelouch of the Rebellion (Game)
Euphemia seems to possess a Geass ability in the alternate story paths in the DS Game.

Another possibility is that she is influenced by Lelouch's (or someone else's) Geass, and that's how it is presented by the developers.

Trivia

 * Euphemia shares a number of character traits with Lacus Clyne, from Gundam SEED and Gundam SEED Destiny, such as similar their peace loving personalities, pacifistic natures, and hair color. Her love interest, Kira Yamato is also similar to Suzaku, both are brown haired pacifists and are the mecha pilots of their respective series. Both are subject to racism (albeit for Kira it is envy as opposed to socially constructed inferiority)
 * The relationship between Euphemia and Suzaku somewhat resembles the relationship between Elie and Haru from Rave Master. Coincidently, those two are voiced by Michele Ruff and Yuri Lowenthal respectively.
 * Her name means "Auspicious speech"  in Greek

Quotes
(To Suzaku Kururugi) "Suzaku Kururugi. The truth is you and I... ah that is... you see... I hereby command you to love me. And in return, I will love you forever. Suzaku, I love your stubborness, and your kindness, and your strength. I love your sad eyes, your clumsiness, and the way you have trouble with cats. I love everything about you. So please, don't hate yourself!"