Imaginary friend
According to Jeanne-Antoinette Poisson, it was customary to have an imaginary friend only during one's childhood. She likened the Tenth Doctor to an imaginary friend. (TV: The Girl in the Fireplace)
The First Doctor had an imaginary friend called Binker. (AUDIO: The Abandoned)
Growing up after meeting him, (TV: The Eleventh Hour, Let's Kill Hitler) Amy Pond knew the Eleventh Doctor as her imaginary friend. (TV: The Beast Below, The Big Bang)
Rory Williams had an imaginary friend called The Chiropractor. (PROSE: Stop, Thief!)
Mels Zucker had an imaginary friend who told her to kill people. (PROSE: Stop, Thief!)
Sandy was Annabel Price's imaginary friend. It came to life. Chris Cwej noted that, for an imaginary friend, it was "not that imaginary... or friendly". (AUDIO: The Dread of Night)
Nardole claimed to have had an imaginary friend, until he left him for someone else. (TV: The Lie of the Land)