In episode 5, Suzaku kills Lelouch with hardly any trouble. CC had already achieved her purpose with him, though. The last kiss wasn't necessary to regain Lelouch's memories. All she needed for that was to will it. The kiss copied Lelouch's brain patterns and stored them. She will end up giving these to Rollo, who will become the new Zero. Rollo, saddened by the death of the brother whom he both admired and restrained himself for, become a much stronger Zero than Lelouch could ever hope to. But he keeps his mask on at all times, and pretends to be Lelouch for the entire show.