First Season

Lelouch is introduced in the first episode of the series as a student of Ashford Academy, where he is a member of its student council. A brilliant thinker who is quite talented at chess, he lives with his younger sister, Nunnally, and their maid, Sayoko Shinozaki, at a house situated within the compounds of Ashford Academy, where they are looked after by the Ashford family.

Lelouch accidentally boards a truck being used by Japanese resistance operatives. Within the truck is a capsule holding C.C., who sacrifices herself to save him from the military forces trying to recapture her. When it seems as if her sacrifice was pointless, C.C. suddenly touches his hand and offers him the "Power of the King", the mythical power of Geass. The Geass manifests itself in him as the power of absolute obedience, which allows him to make people obey his orders without question. He uses this power to order the Britannian soldiers to kill themselves.

With his new power, Lelouch begins his rebellion against the Empire, starting by killing his half-brother, Clovis, after extracting information about the murder of his mother, Marianne. He later forms the Order of the Black Knights and takes up the identity of Zero to lead them, becoming a revolutionary and gaining popular support amongst the people.
The turning point in his rebellion comes when Euphemia li Britannia declares the region under Mount Fuji the Special Administrative Zone of Japan, giving the Japanese people their name and country back, albeit in a much smaller area. This effectively destroys any possibility of the Black Knights' rebellion; acceptance would render them powerless and refusal would make them pariahs. Lelouch confronts her at the opening ceremony and tries to have her shoot him, hoping to make himself a martyr. When she says she plans to give up her royal title, he surrenders and agrees to work with her.

 However, at this moment his Geass permanently activates without him knowing, and an offhand comment about ordering her to kill the Japanese causes Euphemia to do just that. Lelouch reluctantly kills her and uses the massacre as an excuse to spark the Black Rebellion, in which he declares Japan to be an independent nation and leads an attack on the Tokyo Settlement. The attack goes well at first, but when Lelouch learns that Nunnally has been kidnapped, he abandons the battle, leaving his forces helpless against the better-organized Britannian military.
Lelouch makes his way to Kaminejima to search for Nunnally but is confronted by Suzaku. Suzaku exposes Zero as Lelouch to Kallen as she attempts to protect him. Lelouch attempts to broker a truce for Nunnally's sake, but Suzaku is distraught over the death of Euphemia and refuses to listen. Lelouch draws his pistol and both he and Suzaku attempt to shoot one another.

CC

C.C. is introduced in the first episode, where she emerges from a fake gas capsule seized by Japanese rebels. She saves Lelouch Lamperouge from the Britannian soldiers trying to recapture her, taking the shot intended for him, and grants him the power of Geass on the condition that he grant her greatest wish. Lelouch accepts and is granted the power to control people's actions. Since she appears to have died following this, he leaves her body behind.

She unexpectedly reappears in the fifth episode, casually folding origami with Nunnally. From this point on she passively oversees Lelouch's missions as Zero, wanting to keep him alive so he can fulfill his contract, often going as far as wearing his clothes and impersonating him when situations become life-threatening.

When C.C.'s existence is revealed to the Black Knights, she gains considerable unofficial standing, as she is seen by other members to be one of Zero's closest advisers, if not his top adviser. However, her willful attitude and lack of an official posting annoys some veteran members and causes confusion as to her role within the Black Knights. At one point, Tamaki goes so far as to accuse her of being a member only because she's Zero's lover, a claim which she coldly denies. When the Black Knights gain possession of the Knightmare Frame Gawain, she becomes its pilot while Lelouch operates the weapons.

In the season finale, she uses the Gawain to battle Jeremiah Gottwald's Siegfried so Lelouch can save his sister. She kisses Lelouch before she drags both the Siegfried and herself to the ocean floor to end the battle while narrating that the spark of Lelouch's actions was the core of human motivation: the desire for just a small amount of happiness. According to Yukana in a conference memo, C.C. was crushed by the water pressure, but being immortal, ultimately drifted to the surface and revived; it is stated that it was too gruesome to be shown.

Kallen Stadtfeld

Kallen appears early in the first episode, driving a truck with C.C.'s capsule, and then providing cover by fighting Jeremiah in a Glasgow. After Lelouch gains his Geass from C.C., he takes control of an enemy Sutherland and helps Kallen to escape the other Knightmare Frames chasing her. He further directs the remaining resistance forces in the battle, granting them a decisive advantage until Suzaku Kururugi uses the Lancelot's superior systems to overwhelm their forces. She sacrifices her own machine to allow Lelouch to escape, despite not knowing who he really is.

After this incident, Lelouch's unfamiliarity with his Geass causes him to reveal knowledge of the Shinjuku incident to Kallen while questioning her about her motivations, leading her to suspect him of being their unknown benefactor. Lelouch disproves these suspicions by being present while she receives a telephone call from Zero, which actually plays a message he recorded. With the formation of the Black Knights and Zero's increasing number of victories over the Britannian forces, Kallen comes to greatly admire him, evident in her ecstatic reaction to her appointment as head of his personal guard. She even tries to assassinate Suzaku Kururugi after learning that he is the pilot of the Lancelot, believing it would benefit Zero's goal. The trust seems to be mutual, as Lelouch offers to reveal his identity to her when he notices her curiosity; she declines, though she shows signs of jealousy that C.C., whom she had first met in the previous episode, appears to know.

The Guren Mk-II

Once the Black Knights begin receiving new Knightmares, Zero gives Kallen the newly developed Guren Mk-II. With its powerful radiation-emitting arm and speed matching that of Suzaku's Lancelot, Kallen becomes one of the Black Knights' most talented pilots, often facing off against Suzaku and usually proving an even match for him.

At the end of the first season, Kallen reveals her identity to the students of Ashford Academy when the Black Knights occupy it as their base of operations, convincing the student council that they will not be harmed.

 Later on, she battles against Suzaku in his flight-equipped Lancelot, suffering a narrow defeat. Lelouch intervenes before Suzaku can deliver the finishing blow. When Lelouch abandons the battle to rescue his sister, Kallen follows him. She witnesses his confrontation with Suzaku and learns about his true identity and his Geass power. She flees in shock rather than help Lelouch deal with Suzaku.

Suzaku

Suzaku is introduced in the first episode of the series as an Honorary Britannian soldier.[1] While searching the Shinjuku ghetto for poison gas stolen by resistance members, he finds Lelouch standing next to it. He refuses to carry out an order to kill Lelouch, and is shot by his superior officer for his insubordination. However, the bullet is stopped by a broken pocket watch belonging to his deceased father.

Suzaku's Knightmare Frame, the Lancelot
While recovering from his injuries at the command center, Suzaku is approached by Lloyd Asplund and offered a chance to pilot the experimental Knightmare Frame Lancelot, since the pilot that Lloyd had originally been looking for was already on the front lines.[2] The Lancelot's advanced systems combined with his unprecedented piloting skill allow him to almost single-handedly dismantle Lelouch's plan to rout the massacre of Shinjuku ghetto, but he finds himself under arrest afterward for the murder of Prince Clovis; though Lelouch was in fact responsible for this, the military used him as a scapegoat since they had no suspect, citing his Japanese heritage as motive in spite of his Honorary Britannian status.
Lelouch rescues Suzaku from his impending execution by assuming the identity of Zero and taking credit for Clovis' murder, forcing the military to acquit Suzaku due to lack of evidence. He asks Suzaku to join his cause, but Suzaku refuses. While he agrees with Zero on the latter's assertion that the Britannian Empire is corrupt and not worth serving, he wants to change and improve the Empire from within, to show that his father's death was not in vain.

From this point on, Suzaku remains the pilot of the Lancelot. He is reunited with Lelouch and Nunnally after enrolling into Ashford Academy at Euphemia's suggestion. His rank within the military is initially Private, but he is promoted to Warrant Officer by Cornelia li Britannia when she first arrives in Japan.[3] He is appointed as Euphemia's personal Knight, granting him the rank of Major, after a televised battle with the Black Knights makes his occupation public knowledge. This ruins Lelouch's plan to have Suzaku protect Nunnally once his role as Zero took him away from Area 11. During the battle on Shikinejima Island, in which Suzaku is ordered to sacrifice himself to kill Zero, Lelouch is forced to use the power of Geass on Suzaku in order to escape destruction, and commands him to "live," forcing him to survive by any means from then on.
Suzaku falls in love with Euphemia, and thus is emotionally crushed by her murder at Zero's hands. He is visited by V.V., who explains Zero's power of Geass to him, which drives him to find and kill Zero at all costs. He follows Lelouch to Kaminejima and confirms his identity. Lelouch tries to offer Suzaku a truce in order to rescue Nunnally, but Suzaku is too swept up in his grief to care. Enraged, Suzaku insists that Lelouch's very existence is a mistake, one he intends to correct. Lelouch draws his pistol and both men attempt to shoot one another, at which point the screen fades to black and the season ends with a gunshot.