Never let him see you in pain.
The back of my heels felt as though they were on fire, I might have been exaggerating, but I never complained; it would just be rude. Besides, the Doctor would overreact if he found out. So I kept running, but one wrong move sent us flying as an explosion knocked us off our feet, shattering the window behind us and sending shards of glass flying. Some of the glass slipped down the back of my shoes, scrapping up my partially healed scars that had opened up again over a week ago. I gasped in pain, clutching at my heels; ripping away the shoes and slip my socks down to the arch of my feet, trying to get to the now bloody mess. I tried to control my reaction, but it was too late, the Doctor was staring at the bloody mess which was the backs of my heels. I tried to fix myself up, stand, and walk forward, only to fall to the ground again crying out in pain. All the while the Doctor watched silently, only was it until I had tried for the 3rd time to walk normally to walk did he move towards me, pick me up off the ground, and start walking as fast as he could. In his arms I could feel he was tense, and looking up at his face I could see it was hard and unreadable. "Why didn't you tell me Bree?" He said quietly, our eyes met and I felt myself shrink back. His eyes were full of disappointment.
Never let him see you age.
It hadn't been too long since I had seen the Doctor, yet I had seen him as my seven year old self instead of my current 15 year old state. But for him, it had been a very long time, an entire regeneration. He was a whole new man now, and as we faced each other on the sidewalk, 5 meters away from the other; I saw his brown eyes light up in recognition, only to see only moments later his eyes and body language fall in response as he looked me over. I was no longer the 7 year old he had come to know, trust, and adore. No, I was a teenager now. I had grownup. That was the thing though, he knew I was a time traveller, he knew I would grow up eventually. Just not that fast. He could just be so emotional at times. He had changed too, and just as quick, from blue eyes and leather jackets, to wild brown hair and eyes with long overcoats over suits, he was completely changed. A whole new man. He could become someone entirely new, while I wasn't allowed to grow up.
Never let him see the end.
The fire faded, my own "regeneration", coming to an end. But I didn't really regenerate, the other half of my nature took over; my vampire side was saving my life. For once I wished my vision wasn't crystal clear; the Doctor's green eyes bore into my temporary red ones, full of shock, sadness… and disappointment. As a human, I had breathed my final breath until the vampire side would heal the internal wounds, but that wasn't good enough for him. To the Doctor, I had basically, just died. Returning to human within a couple of hours did not matter, to him; I was already dead.