Kaguya Sumeragi (皇 神楽耶, Sumeragi Kaguya) is the young head mistress of the Kyōto House, and a supporter of the Black Knights. Kaguya is Suzaku's cousin, and met Lelouch and Nunnally during their childhood at the Kururugi Shrine. She is the representative of Sumeragi Concern (皇コンツェルン, Sumeragi Kontserun), a large conglomerate founded by the Sumeragi House. Despite her age and physique, she possesses a strong will and is highly influential amongst the Houses. She is named after Kaguya, the main character of what is considered to be the oldest Japanese folktale.

Kaguya has admired Zero since his debut, and wishes to have his hand in marriage. She claims to be his wife in the second season, though she admits that they are only husband and wife in name. She doesn't mind him sleeping with other women; she claims "a true hero enjoys physical pleasures," and proceeds to suggest that she, Kallen, and C.C. are Zero's harem.

Kaguya is the only living member of Kyōto following the Black Rebellion, having managed to escape to the Chinese Federation until Zero's return one year later while the others were executed. With the formation of the United Federation of Nations, she is made the Supreme Council Chairwoman and the representative of the United State of Japan. During the assault on Area 11, Kaguya is present on the Li Xingke's command transport. After receiving word of Zero's death and the armistice between the Black Knights and the Britannian forces, Kaguya returns to the Ikaruga to negotiate the armistice terms. She is present when news of a rebellion within the Emperor's forces near Kaminejima arrives. Believing that only one person would dare to attack the Emperor at such a time, Kaguya decides to follow Schneizel to the island. She also postpones their meeting, needing to confirm the details about Zero's death. She is told the truth by Ohgi.

After Lelouch installs himself as Emperor the following month, she chairs Britannia's entrance into the U.F.N. She and the others easily see through the fact that Britannia's large population would give it a supermajority vote in the U.F.N., effectively giving Lelouch control of the organization, and try to get him to limit his vote. Lelouch instead takes her and everyone else present hostage, bringing Kaguya to tears. However, she questions his motives when Lelouch abruptly leaves the situation, and she is eventually freed with the rest of the hostages. Because of this, she sees Lelouch's true mask and part of his true nature, and understands him, which causes C.C. to call Kaguya a kind person.

Kaguya is later kidnapped by Lelouch, and she is slated for public execution along with Tianzi, Xingke, Ohgi, Kallen, Schneizel and others. They're all released when Lelouch is killed by Suzaku (dressed as Zero), and later she attends Viletta and Ohgi's wedding.

Source: Wikipedia