Barnable

Barnable was a boy who lived in the town of Christmas on the planet Trenzalore three hundred years after the arrival of the Eleventh Doctor. He was a close friend of the Doctor and believed in him. When a wooden Cyberman arrived, he alerted the elderly Doctor and watched as he destroyed the Cyberman. Barnable thanked the Doctor for fixing his father's barn and asked him why it was now bigger on the inside, but the Doctor asked him to keep it secret. When the TARDIS returned and the Doctor started to leave for the Papal Mainframe, Barnable hid himself behind the TARDIS and sadly asked the Doctor if he was leaving for good now. When the Doctor told him he would return, Barnable promised to wait for him. Barnable was still waiting when the Doctor returned some time later. (TV: The Time of the Doctor)

Later mentions
The Doctor called Aliganza Torp "Barnable the 43rd" when the latter was a child. After the original Barnable died, the Doctor took to calling all the children he liked after him. (PROSE: The Dreaming)

600 years after Barnable's time, the now-frail Doctor believed that Clara was the presumably-long dead Barnable when she arrived and later asked another person if he was Barnable, showing signs of senility and his fondness for the boy by still remembering him in that state. The young man said no, but the Doctor called him Barnable anyway. (TV: The Time of the Doctor)

Behind the scenes

 * Barnable was supposed to appear as a hallucination when the Eleventh Doctor regenerated, along with other of the Doctor's companions. In the end, only Amy appeared.