Zero

Zero is a public masked identity, originally created and assumed by Lelouch vi Britannia. Although for a long time Lelouch remains the person behind the mask, throughout the conflict between Britannia and the Order of the Black Knights, Zero and his distinctive appearance remain a symbol of the anti-Britannian rebellion and the fight for an independent Japan.

Emergence
Lelouch made the original Zero costume by using his Geass on a craftsman to custom-make it for him and destroy the evidence. In this capacity, he first appeared to the future Black Knights in person, keeping his face and real name concealed. Zero then made his public debut during the operation to free the framed Suzaku Kururugi, revealing himself as Clovis la Britannia's killer.

Zero's fame quickly spread around the resistance groups throughout Japan. When Lelouch announced the Order of the Black Knights to the world, he remained its undisputed leader as Zero until the Black Rebellion, with C.C. sometimes donning the costume to act as a decoy for him. Zero became a symbol in Japan, a messianic figure, especially after Euphemia li Britannia was accidentally turned by Lelouch's Geass into an enemy of the Japanese and ordered their massacre.

During the first assault on Tokyo, however, Lelouch left the Black Knights behind to seek his kidnapped sister, Nunnally vi Britannia. In his absence, the Black Rebellion crumbled, and Britannia declared Zero dead.

Return
After C.C. restored Lelouch's memories, previously meddled by Emperor Charles zi Britannia when he was captured by Suzaku, Lelouch once again assumed the Zero identity, announcing his return to the world and freeing and reforming the Order of the Black Knights. For a long time, many higher-ups in Area 11 did not believe him to be the same Zero who led the Black Rebellion, including Suzaku, who knew Lelouch was the previous Zero but believed he still did not have his memories returned.

For a brief time during Lelouch's depression over Nunnally vi Britannia's plans to recreate the Special Administrative Zone of Japan, Kallen Kozuki considered taking over as Zero, with C.C. remarking how metaphorically heavy the mask was. Lelouch finally returned to his role and came up with a new plan to move some of the Japanese out of Britannian control. Per this plan, Zero negotiated his exile from Britannia in exchange for assembling one million people to participate in the Special Administrative Zone. However, what he actually did was distribute one million Zero costumes among the participants, thus making them all fall under the provision. These "Zeros" were evacuated to Penglai Island, which became the new headquarters of the Black Knights, having assumed the function of the United States of Japan's government-in-exile.

Some time prior to the creation of the United Federation of Nations, Zero became the official CEO of the Order of the Black Knights. However, after the second battle of Tokyo, Schneizel el Britannia revealed the truth about Zero's real identity and his use of Geass to the Black Knights, causing them to turn on him. Lelouch fled from the Black Knights and Schneizel, rescued by his "brother" Rolo Lamperouge, and abandoned the Zero mask, acting publicly as Emperor Lelouch vi Britannia upon his ascension to the throne one month later. The Black Knights officially hid the truth about Zero's identity and escape, declaring him dead from his injuries in the battle.

Passing of the torch
In three months after Lelouch established himself as Emperor of Britannia, he built up a public image of a tyrannical enemy of the world. It was then that Suzaku Kururugi appeared in the Zero costume and assassinated Lelouch, thus fulfilling their plan for Zero Requiem. Per Lelouch's wish, Suzaku &mdash; officially believed killed in the assault on Damocles &mdash; continued his life as Zero, now considered savior of the world, unable to show his face. As the second Zero, Suzaku remained at Nunnally's side; and Schneizel, previously forced into serving Zero by Lelouch's Geass, turned his loyalty to the new Zero after the Emperor's death.

Quotes (as Lelouch)

 * "People of Japan and all who are oppressed by the empire of Britannia, long have I waited, all through the struggles against Britannia's injustice, I waited for them to come to their senses, but that hope was betrayed, by an act of barbarism that can only be called genocide. ... Euphemia is the symbol of Britannia's hypocrisy, a murderer cloaked in the flag of a nation. ... I hereby declare our independence from Britannia, but don't take this to mean the resurrection of your fallen nation, we will not turn back the hands of time, the new japan we go on to build, shall be one broad enough to accept all peoples, histories, and ideologies. Where the strong shall not rein over the weak, and it shall be called The United States of Japan."
 * "Yes, I am Zero, the man who leads the Black Knights, who challanges the Holy Britannian Empire; The one who holds the entire world in his hand."