li Britannia, Euphemia

ユーフェミア・リ・ブリタニア

Euphemia li Britannia (ユーフェミア・リ・ブリタニア, Yūfemia RI Buritania) was the Third Princess of the Britannian Imperial Family. Euphemia is Lelouch vi Britannia's half-sister and the sister he cared most about after Nunnally. Euphemia served as the Sub-Viceroy of Area 11, a role she admittedly did not enjoy. Euphemia admired her older sister, Cornelia, but despised all combat in general, though she could pilot a Knightmare Frame.
Although Euphemia is a member of the Britannian royal family, she remains kind-hearted and open-minded to everyone around her. She treats everyone with respect, even if they are not Britannian. This universal respect probably stems from her childhood experiences with her siblings Lelouch vi Britannia and Nunnally vi Britannia.
She is the second person in the Imperial Family to discover that Lelouch is alive. The first to know his alter ego is Zero, having deduced it after the hotel hijacking incident. In her childhood, she was pretty close to Lelouch and Nunnally. She develops a close, romantic bond with Suzaku Kururugi because of their shared belief.
First Season:

Spoiler

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 Suzaku and Imperial Family calls her. They become friends, and shortly after meeting, she reveals 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, Euphemia attempts to use her position to free the other hostages. In his Zero persona, Lelouch has incidentally led the Black Knights to the exact location, and the two meet. 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 ideals and Cornelia's advice, Euphemia chooses Suzaku Kururugi. However, Cornelia warned Euphemia that her plans might cause her trouble and opposed her choice. Regardless, Euphemia sticks to her decision, and Suzaku remains her knight; until he voluntarily steps down after being 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. However, an encounter with Nina allows her to discover that she has developed feelings for Suzaku. At that moment, Suzaku engages in a battle with a new prototype of Lancelot. Subsequently, his energy level drops, and Euphemia makes 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 he alter ego a secret.
During the visit to the Ashford Academy School Festival, the media catches her visit to the school. Suddenly, she makes a surprise announcement, declaring the Special Administrative Zone of Japan for the Japanese people. This decrement runs utterly 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. He supports her idea and promises to inform Cornelia of it.
When he meets her as Zero, Lelouch reveals his opposition but becomes convinced of her conviction when she tells him she is doing it for the sake of Nunnally. She has surrendered her right to succeed the Britannian throne. Upon agreeing to Euphemia's proposal, Lelouch goes off on another conversation with Euphemia, eventually saying that he could get her to do anything he asked for if he wanted to. He jokingly mentions killing all of the Japanese people. 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 the innocent crowd of people. Lelouch then ordered to stop Euphemia from the massacre of all Japanese and quoted: "Stop the Britannians! Save the Japanese!" He suddenly becomes teary-eyed as he called, "...and kill Euphemia!" Lelouch, with much regret, fatally shoots her in the stomach to end her rampage while revealing that he considered Euphemia his first love.
Suzaku recovers Euphemia's body, and she spends her final moments with him. As he is of Japanese descent, the command to kill him 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. Realizing she has little time, Euphemia urges Suzaku to stay in school for both of them and dies smiling, telling him how happy she is.
Her death creates a firm resolve in both Suzaku and Cornelia to defeat Zero. Lelouch is also profoundly affected by Euphemia's death and resolves to fight and defeat Britannia once and for all.