Suzaku Kururugi is a childhood friend of Lelouch and Nunnally, whose father was the Prime Minister of Japan. He met the two when they were sent to Japan as political hostages. Thinking that it would stop the war, Suzaku killed his father, which haunts him day and night. As repetence, he joined the Britannian army, and the next time he was seen was in the back of a terrorist truck, searching for the "Poisonous gas". When he refused to kill Lelouch and the girl in the capsule, the commander shot him.
Suzaku was thought to be dead but he was alive, and he was saved by Lloyd, who had him pilot the Lancelot, the newest knightmare frame in development. Even though he was a test pilot, his skills were undeniable.
Because he is an Eleven, he was convicted of killing Prince Clovis. As he was about to be executed, he was rescued by Zero, because Lelouch wanted to save his friend. Zero announces that he is the one who killed Clovis. Even though Zero saved his life, he refuses to join Zero, saying that it is wrong, and he will change Britannia from the inside. Subsequently, he was cleared of all charges.
Later, he met a girl from the mainland named Euphie, who jumped out of a window, and he happened to catch her. Without knowing why, he gave her a tour of Area 11, having no idea of her identity. When the purebloods attacked Jeremiah, Suzaku saved him, the very person who accused him in the first place. When the fighting got out of hand, Euphie jumped in, announcing that she is the 3rd Princess Euphemia, viceroy of Area 11. Under Euphie's recommendation, he was enrolled into Ashford Academy, where he reunited with Lelouch and Nunnally. At first, he had no friends, because he was an Eleven at a Britannian school. But after he saved Lelouch from falling off a roof, trying to catch Arthur, Lelouch declared that he is Suzaku's friend, and subsequently he is invited to join the student Council.
When Nunnally was kidnapped by Mao, Suzaku was the one who helped Lelouch rescue her, because he needed Suzaku's superior phsyical prowess. Like many knightmare pilots, he is extremly quick and fast on his feet, as seen by his ability to dodge bulelts from a gun turrent inside the academy. Even though he was not accepted by many because of his Eleven status, he still does what is right. In the battlefield, he came to Princess Cornelia's rescue, who was grateful for what he did. He even was chosen to become Princess Euphemia's personal knight.
Unfortunately, he did not make it in time to save Euphie was she was killing the Japanese. By the time he got rid of Kallen, she was already shot, and upon her death, he swore to get even with Zero. He finally found Zero inside a cave, as he was trying to find Nunnally. He shot off Zero's mask, exposing his true identity, and Kallen was just as shocked as he was.
At the beginning of season 2, it is revealed through Lelouch's memories that he sold Lelouch to Emperor Charles in order to gain his Knight of Rounds status. When Suzaku was sent back to Asford Academy with his friends Anya and Gino, he was determined to find out if the new Zero is Lelouch. Suzaku does find out later on, that Zero is indeed Lelouch in disguise, and he has been fooling him this whole time.
Spoiler:
Suzaku has been Lelouch's antagonist from the very begining, however, when Lelouch became the emperor, Suzaku joins him in controlling the world. By massacring many people, they hope to clear Euphie's name of the Massacre Princess. He was supposedly killed by Kallen in the final battle, but he survives (presumably because of the live on geass) and is wearing Zero's custome, and kills Emperor Lelouch. By doing so, he has cleared Euphie's name, and destroyed the symbol of hatred of the world.
