Euphemia li Britannia was the daughter of Emperor Charles zi Britannia and the younger sister to Cornelia li Britannia, making her a princess of the Holy Britannian Empire. She was very close to Cornelia, who doted over her as children and continued to protect Euphemia as an adult. The two were also rather close to their half-siblings, Lelouch vi Britannia and Nunnally vi Britannia. The four of them spent much time at Aries Villa, the garden of Lady Marianne - Lelouch and Nunnally's mother.
As a teenager, Euphemia was a student in Britannia.
Euphemia eventually made her way to Area 11, the land formerly known as Japan. When her half-brother, Prince Clovis la Britannia, was assassinated, her sister Cornelia was brought in to restore order. Euphemia, in turn, came with her, forcing her to leave her school. Euphemia was officially made the Sub-Viceroy, handling small, local affairs while her sister, Viceroy Cornelia, led a military campaign across Japan hunting for Clovis' killer - Zero.
Euphemia stayed in hiding for awhile, defying her guards and escaping into Tokyo. As she escaped, she met Suzaku Kururugi, the man who was framed for killing her brother, but was let go when the true killer revealed himself. She called herself "Euphie," preferring to keep her royal status a secret. Lloyd, an earl who led the Research and Development department that was primarily under the command of Prince Schneizel, decided to follow Euphemia, keeping a distance to make sure she was safe.
Euphemia and Suzaku went to the Shinjuku ghetto, the location of the massacre performed by Britannia that was covered up as an accident. While there, they witnessed an assassination attempt on the life of Jeremiah Gottwald, a Britannian soldier who led the investigation into Clovis' death, but mysteriously let the suspect escape. Lloyd revealed his presence to Suzaku, giving him the Lancelot Knightmare - which he kept in his truck - to intervene in the battle. During the fight between Suzaku and the would-be assassins, Euphemia entered the fray, revealing that she was Princess Euphemia, and ordered all combatants to stand down.
Euphemia told Suzaku to enroll in the Ashford Academy. She wanted him to finish his education, because she was not able.
Euphemia went undercover as a student during the Hotel-jacking that was performed by a small group of Japanese loyalists. Euphemia, posing as a student, was taken hostage along with students from Ashford Academy. When one of the terrorists threatened to beat one of the girls - Nina Einstein - Euphemia revealed herself in order to protect the girl, despite her adviser pleading with her to remain in hiding. However, when she was brought before the terrorist leaders, they were all found dead - killed by Zero, he half-brother's killer. For reasons she did not understand, she was allowed to live, despite her royal status.
The hotel collapsed during the incident due to attacks performed by the Britannian military - which were strategically led by her sister, and actively performed by Suzaku. However, Euphemia and the hostages were rescued by Zero, who used the attack as a public revelation for "the Black Knights," a group of former terrorists who sought the downfall of Britannia.
Euphemia kept to small affairs while her sister, Cornelia, strengthened her efforts to kill Zero and the Black Knights. After the Battle of Narita, where Cornelia's forces were beaten by Zero, Cornelia told Euphemia that she should choose somebody to be her knight - for protection and because, as royalty, she should have a knight. While she preferred to have Suzaku as her knight, it was not acceptable for her to have an Eleven - Japanese - man be her knight.
Euphemia put off choosing her knight for as long as she could. During a gallery exhibition, where Euphemia had to choose from Britannian paintings to display, Zero's Black Knights freed General Tohdoh, a Japanese loyalist involved in the original war against Britannia. The fight was displayed on the news, showing Suzaku fighting against the Black Knights. During the press conference, Euphemia shocked everybody by publicly proclaiming that she would, against the negative opinions of her sister and Britannian citizens, choose Suzaku Kururugi to be her knight.
Euphemia, accompanied by her new knight, went to Shikine Island on a diplomatic meeting. There, they were attacked by Zero and the Black Knights, who intended on capturing Suzaku and his Lancelot Knightmare mech. During the attack, Suzaku is trapped in a field that causes all mechs to stop functioning. In the scuffle, Suzaku manages to get a hold of Zero. Euphemia is alerted to the fact that orders from a Britannian royal command that Zero be killed, sacrificing Suzaku. Euphemia enters the fray in an attempt to stop them from firing, but they do not, causing a large explosion in the area.
They are not killed, however. They were all mysteriously teleported to an island away from the battle. The four of them - Euphemia, Suzaku, Zero, and Kallen Kozuki (one of Zero's Black Knights) - ended up on separate ends of the island. Euphemia came across Zero, who threatened her with a gun. Euphemia, however, causes him to stand down when she reveals who she thinks he is. She asks him to take off his mask, and he does, confirming her suspicions - that Zero is, in reality, Lelouch vi Britannia, Euphemia's older half-brother and former prince.
The two sat and talked about old times, back when the two of them were children. Lelouch confirms that Nunnally, his sister and Euphemia's younger half-sister, is alive and well. Meanwhile, across the island, Suzaku and Kallen begrudgingly work together to survive in the wilderness.
They were only on the island for a short time, however. Lelouch and Kallen escaped, while Euphemia and Suzaku were rescued by Britannian forces - which included the one who fired on the island, her half-brother Prince Schneizel el Britannia.
Afterward, Suzaku was investigated for attempting to leave the battlefield during the initial engagement; he was shown a recording of him shouting that he wanted to live above all else. This was a mystery to Suzaku - who did not remember doing that - and Euphemia, who thought that was extremely out of character. While Suzaku was not arrested, he felt that the event made him unworthy of being Euphemia's knight, and stepped down.
Meanwhile, a faction of exiled Japanese politicians - allied with Chinese officials, who acted without the direct consent of the Chinese Federation - invaded Japan, setting up a base. Suzaku was sent in to take them down, but his prototype flight unit drained his power. Over the intercom, Euphemia pleaded with Suzaku, and confessed her love for him. At his darkest moment, when Suzaku was almost dead, he was rescued - by none other than Zero. Zero and Suzaku, for the first time, worked together to bring down this rebel faction, who was not working in the best interests of the Black Knights or Britannia. Euphemia was overjoyed that Suzaku and Lelouch could work together. After the battle, Suzaku resumed being Euphemia's knight.
The Special Administrative Zone of Japan
Euphemia went in secrecy to the festival being held at Ashford Academy, where Lelouch and Nunnally lived. Lelouch was initially horrified, fearing his secret. But Euphemia promised to keep his identity and existence secret, wishing only to see Nunnally. Lelouch, Nunnally, and Euphemia talked about their future, and Euphemia claimed that she had a way to fix everything. At one point during the festival, the crowd noticed Euphemia, and mobbed her. She was, however, rescued by Suzaku. She took control of the situation, asking for a news broadcast. It was at that moment that she made a groundbreaking announcement: she would create the Special Administrative Zone of Japan - a place where Elevens would be allowed to call themselves Japanese, speak Japanese, and act Japanese, granting them freedoms that they did not have for eight years. This horrified Lelouch, who saw it as a sinister move to destroy the Black Knights.
Euphemia was chastised by Cornelia for making the decision without talking to her, but allowed her plans to move forward. Millions signed up to the Special Zone within days. Euphemia openly asked Zero to join her in her efforts, as well as other Japanese who had been exiled.
She met with Zero during a ceremony, the two retreating to where they could talk in private. Lelouch told her his intentions: he wanted her to shoot him, making him a martyr in order to keep the Black Knights in action. Euphemia did not wish to do that, and thought that she and Lelouch could work together to create the Special Zone. He got angry, pushing her away. In an act of anger, he began to order her to shoot him. But before he could, she proclaimed that she planned to renounce her royal status. That shocked Lelouch, who finally saw her sincerity and genuine compassion. Lelouch agreed to work with Euphemia in order to maintain the Special Zone.
However, unknown to Lelouch, his power known as Geass - the supernatual ability to command anyone to do anything he commands - had become uncontrollable. Euphemia asked how he thought that she would submit just because he asked something of her; he explained his powers, giving her the example that he could compel her to do anything, including killing every Japanese citizen. His Geass activated with that order. Euphemia tried desperately to resist, collapsing to the ground. But, within seconds, she submitted. She ran from him back to the ceremony ground, ordering her soldiers to kill every Japanese person in the stadium. She, personally, shot a Japanese man. The slaughter claimed countless Japanese civilians.
Zero called in the Black Knights to restore order, and they proceeded to fight Britannia and protect the Japanese. Euphemia herself went on a mindless killing spree, picking up a gun and, later, entering a Knightmare to kill any fleeing Japanese. Suzaku Kururugi pleaded with her to stop, but she did not hear him. Zero disabled her Knightmare frame, entering the battle to end the conflict. Euphemia momentarily realized what was happening, and appeared confused. Zero turned and shot Euphemia through the heart, proclaiming that she was the first girl he ever loved.
Suzaku, enraged, entered the battle in the Lancelot, scooping up the injured Euphemia. He retreated back to the Avalon, the mobile Britannian command center. He brought her to the medical center, but the doctors claimed that it was too late for her. Suzaku stayed by her side as she slowly slipped away. For a moment, Euphemia had a desire to kill Suzaku due to the residual effects of the Geass, but she resisted it. Without memory of what happened, she asked Suzaku if people were happy; to which Suzaku lied, putting on a face of happiness so that Euphemia died content.
Cornelia did not arrive in time to see her sister before she passed away. Cornelia was in shock for some time, coming out during the Battle of Tokyo in a blood rage, relentlessly attacking Zero for what he did.
Euphemia was a focal point of hatred for Elevens / Japanese, used by Zero and the Black Knights to rally against Britannia. Even a year after the incident at the Special Zone, she was remembered for her actions. She became known as the "Massacre Princess."
Suzaku turned all his efforts toward killing or capturing Zero, which he eventually succeeded in. However, Suzaku always found it curious that Euphemia, even as she lay dying, never revealed Zero's true identity.
Cornelia became hell bent on destroying Zero. After finally encountering him and finding out his true identity, Cornelia turned her attention toward Geass, the power that was used on Euphemia to cause the massacre. Cornelia relinquished her royal ties, going into self-imposed exile on a quest to find the mysterious Geass Order.
Euphemia is very compassionate and selfless by nature. Because of this, she was deemed weak by most of her siblings.
She is extremely underestimated by many people. However, when she puts her mind to something, it is extremely difficult to deter her. She was able to defy many of her advisers during her time as the sub viceroy of Area 11.
On more than one occasion, she put herself in harms way to protect somebody else, even strangers. She revealed her royal status just to save a school girl from a group of Japanese terrorists.
Unlike many of her siblings, she had almost no combat experience - whether as a commander or fighter.