Kirihara was the guy who they met in Mt. Fuji.
He was a guy who had a monopoly on Sakuradite mining and so the Britannians let him live in exchange for his proactive cooperation. However, he was secretly funding the resistance movement.
Lelouch, Kallen, Tamaki and Ogi all go to see him. They don't know who he is, as his face is hidden. Kirihara orders Ogi to take off Zero's mask. Ogi complies and it is actually C.C. in the costume. She identifies Kirihara by name, and the guards are about to kill the lot of them when Lelouch reveals his presense within one of the Knightmares used by the guards. He takes out the other three Knightmares and then points his Knightmare's weapon at Kirihara, and proceeds to tell him that he is weak and senile.
He then gets out and shows Kirihara that he is actually Lelouch vi Britannia, gaining Kirihara's full support. They commiserate on how the "flower from 8 years ago" has sprouted and how it will destroy Britannia. Kirihara asks if Lelouch will embark on the "path of carnage" and Lelouch responds casually with "if that is what is required of me.
The singularly most badass scene in the entire show. And the next scene ruined the mood it set by having Shirley baaaaaaaaaaaaawwwwwwww about her dead father.