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)

Amy Pond knew the Eleventh Doctor as her imaginary friend. (TV: The Beast Below, The Big Bang)

Nardole claimed to have had an imaginary friend, until he left him for someone else. (TV: The Lie of the Land)