Let me quote a little part from the best Clow/Yue fanfic I ever read 

“Master?” I called softly and I saw him sigh. “What’s wrong?”

“I’m deciding what to do about something,” he whispered. “I think I have failed you.”

I sat up on the bed and focused on him for a moment. He looked torn and then his brows narrowed as he seemed to come to a decision.

“Yue, what are you?” he asked as he focused on me. The gaze seemed to tear through me and I looked away.

“Answer me,” he ordered softly and I felt the magic that made up my soul demand a response.

“A guardian,” my voice sounded hoarse as I admitted it and for a moment I wondered why.

“What am I?” the same commanding, calculating voice.

“Master,” I intoned and then I saw what he was getting at and stared at the object around Clow’s neck. His key. That I had taken from him. “There’s going to be punishment, isn’t there?”

Clow met my eyes and said nothing.

“You hurt me first,” I said softly.

“That’s not the point, Yue. Don’t you understand? I may love you with all the realm of my heart, but you are still a guardian and I am still the Master. There is nothing that can change that. I am sorry that all of this had to happen to being with, my jewel,” he said slowly. “I hurt you, yes, but I also made you. You are my creation. Whether I love you or hate you is of no consequence, you are mine. By my magic alone do you live and breathe, Yue.”

Legacy of the Master chapter 10. by Yakusoku