Because of her death, little is known about Marianne, besides the fact she was the mother of Lelouch and Nunnally vi Britannia, Imperial Consort of the 98th Emperor of Britannia, and a former member of Knights of the Round. She was an aesthetically beautiful woman, and as early Code Geass side materials portray her, sweet and kind hearted, with a rebellious spirit and a deep protective instinct towards her children. Lelouch gives two examples of this: in one she rode a horse into the palace's great hall and in the other she picked a fight with several of the Emperor's other wives all at once. In a flashback from one of the novels, she uses the Ganymede to intimidate a fellow consort who had been harassing Lelouch and Nunnally — all the while maintaining her friendly disposition. Later, side materials reveal her true nature as a selfish, heartless former member of Knights of the Round who helped to overthrow the previous Emperor of Britannia and install Charles as the 98th Emperor. She only superficially loves her children and will not hesitate to kill them if they get in her way.
Marianne's conversations with C.C., one-sided in the anime but scripted out by the staff (and actually revealed in the Nintendo DS game), show that she has a playful side, often teasing C.C. when the immortal girl tries to deny any sort of attachment to Lelouch.
Marianne vi Britannia is the late 98th Empress of the Holy Britannian Empire and the mother of Lelouch and Nunnally. Her commoner status by birth caused her to earn the jealousy of other imperial wives and nobles, which Lelouch suspected had resulted in her murder. Lelouch's investigation revealed, however, that several members of the Imperial House not related to her by blood respected and admired Empress Marianne, such as Clovis, Euphemia, especially Cornelia, who idolized her and put a lot of fruitless effort into investigating the assassination. Even some Britannians outside the Imperial Family, such as Jeremiah Gottwald, respected and admired Marianne.
It is hinted that her supporters were the Ashford Foundation, which explains why their social statuses were ruined after her death and why they consented to protect Lelouch and Nunnally. She had also piloted one of the earliest Knightmare Frames, the Ganymede, which was developed by the Ashford family and is well known for her superior Knightmare Frame piloting skills, hence the nickname, "Marianne the Flash". She also became a member of the Knights of the Round thanks to these skills. Several times throughout the series, Marianne's consciousness has been shown to be communicating with C.C. as a side effect of her Geass.
First SeasonAt the end of season one, Lelouch, by questioning Cornelia using his geass, discovers that Marianne had ordered Cornelia to shut down the security on the day of her attack, implying that she knew about the attack beforehand. It is eventually revealed that the security was lowered by Marianne under the request of V.V. and he was the real culprit, who killed Marianne out of jealousy.
Marianne's spirit is revealed to be within Anya Alstreim's body. She recovers C.C.'s memories and by using the Mordred heads to Kamine Island with C.C. to help Lelouch. While making their way to Lelouch and Charles, they encounter Bismarck Waldstein who is currently engaged in battle with the geassed reinforcements of Shinkine Island. Marianne did not want to waste time and so tricked Bismarck by questioning his loyalty. Knowing that he wouldn't question hers, Bismark believed her, allowing her to lie saying that she will intercept the rebel forces from the ground. C.C. acknowledges Marianne's ability stating that, her nickname "Marianne the Flash" suits her and being the mother of her son Lelouch. She is revealed to have had a contract with C.C., though she never fulfilled her end of the bargain by becoming immortal through acquiring the Code. She attempts to aid Charles in triggering Ragnarök, but is killed when Lelouch uses his Geass to turn the gods on them both.
Marianne's Geass allows her to transfer her spirit (along with her Geass) into another person. A side effect is that, when her consciousness surfaces, she can speak with C.C telepathically. She used it after being mortally injured by V.V.. If her body was kept intact, there was a possibility that she could have returned to it.
Appearances in Other Media
The light novels reveal that Marianne was driven to greater insanity by her Geass than even Charles had been. She experimented on many people using her Geass, including her own children. After the great success she experienced while testing on Lelouch and Nunnaly, she considered wedding the two to create a child who would be the perfect container of the Code.
Nunnally in Wonderland
In Code Geass: Nunnally in Wonderland, Marianne plays the role of the Queen of Hearts. When the King of Hearts (Charles zi Britannia) refuses to let Nunnally go through the castle to get to the path that will allow her to leave Wonderland, the Queen of Hearts appears along with the Knave of Hearts (V.V.). She agrees to let Nunnally go through the castle, but on one condition: Nunnally must play a game of croquet with her. In addition, if Nunnally loses, she will lose her head. However, this is interrupted by the arrival of the White King and Queen (Cornelia li Britannia and Euphemia li Britannia respectively) and the Red King and Queen (Schneizel el Britannia and Kanon Maldini respectively), with both the White Queen and the Red King expressing their different views on croquet. While this is happening, Nunnally manages to slip away and continues towards the exit.