C.C. is the first to notice the subtle changes in the Britannia Prince. This was only naturally seeing as she has purposely made herself his shadow to keep him alive—for the sake of the contract. At first, she simply shrugged and thought it was no different from how everything usually happened, constantly changing and shifting while she stood still and watched them move farther away. It wasn't until they had returned to his house after a meeting with the Black Knights and he had simply walked up to his room rather than check in on Nunnally. The witch had teased him when it seemed that he had forgotten but was stunned with the indifference in his voice when he said he couldn't be bothered with such trivial things. He didn't even bother to turn around to say this and missed the look of fear on the immortal's face.
As time went on, Lelouch became more detached from those around him. The genuine kindness that he would subtly show towards those he cared for was gone. He stopped showing up to the school council meetings, and would ignore any and all calls from them. Shirley, whom he had a certain fondness for was ignored. The last time he talked to her, C.C. could vividly recall the look of humiliation and tear stains as she walked away. Lelouch had simply told her that he was helping her realize how pathetic she acted. One by one, his group of friends began to drop away, the only one ever offering him a single word being Milly, and that was no more than a greeting.
All he had at that point was his sister, who he rarely talked to or about, and the Black Knights. And C.C.. But even then, he stopped holding meetings. He would hand out orders and direct people on what to do, but his over-the-top speeches that would inspire the Black Knights were gone. At that point, it seemed that the young warlock had lost all his motivation help the Japanese and became indifferent. She couldn't help but think of how furious the original Lelouch would be at himself. Occasionally, C.C. could see the look of regret along his face when he was alone in his room and that kept her hoping that this would be a phase that would pass as well.
She knew what the effects of Geass were, and knew how they could be destructive. The memories of watching that power slowly take over Mao never faded, but Lelouch wasn't like Mao. Lelouch was strong, passionate, and intelligent. He would never let the power of the Geass block him from his goal of creating a better world. The Witch would repeat this to herself often, and found little comfort in the empty words. It did little good when she was watching him slowly deteriorate in front of her. "Zero" was slipping. His plans had become more hasty and reckless, and often there was very little to be gained from the plan. The Black Knights had taken notice quickly and it wasn't long before rumors of disbanding the organization or perhaps taking the position of Zero away from Lelouch and giving it to another. Many other plans and ideas the C.C. had shared with Lelouch and he would simply laugh.
And one day, his laughter turned into choked sobs. The raven haired boy's features had twisted itself into an awful grimace as he began to scream and through the helmet across the room. A pale hand had reached out to the witch's own hand, and he pulled her closer. That night, he held her close and whispered apologies for everything. What he had done, who he had become, and what he was going to do. If C.C. were kind, if she were selfless, she would have spared him from going on any further. However she was not kind; instead she held him close to her and prayed that this would end soon and the young Britannia Prince that sought justice and fairness would come back.
Two weeks later, the Geass sigil was in both of his eyes and the original Lelouch hardly ever visited. Every day, the immortal would stand beside the man's body and watch this stranger misuse it, but she'd stay quiet and continue watching to make sure it was safe and whole just in case he would return.
It wasn't until she was standing outside, staring out at the former Empire and watching the people in the streets rush through the streets, looking around nervously for any tanks or soldiers that might have been sent out on a random whim of the tyrant that was once their prince, that she decided that she was still his ally and that she owed him one thing at the very least.
C.C. knocked on the doors of his room, said her name, and walked in. And Lelouch looked on and thought nothing of it. Even as she asked him to come closer to her after closing the door, he complied, because she was his witch, his one and only ally who had done nothing but look after him after forming the contract. And that was what she was doing.
The gun shot was louder than anticipated, and the witch nearly jumped at the recoil. Nevertheless, she kept a steady grip on the weapon to insure that it would only be one shot. C.C. hadn't wanted to use a gun, but it was the most direct and quickest option and Lelouch deserved that. He didn't need to feel his body slowly shut down from poison racing through his veins, or the slow twist of a knife in his chest. This was the best option for him, and so C.C. would bear with the blood spattered along her arms because this was all her doing and she had no right to complain.
As the Geass sigils in his eyes faded, so did the look of shock. Tired but kind violet eyes stared at her as he breathed a sigh of relief. The look of gratitude on his face made her sick, but she did her best to return the look with a halfhearted smile. Even when he collapsed on to the floor and his vision was already gone, she still tried to keep that broken smile.