Nardole's imaginary friend

Nardole once had a male imaginary friend.

After learning that Bill had created an imaginary version of her mum, Nardole revealed that he had also had an imaginary friend. He stated that this friend "left [him] for someone else", which he sarcastically said was "charming". Given Nardole's tendency towards humour, particularly in tense or uncomfortable situations, and also given the unlikelihood of an imaginary friend transferring from one person's imagination to another, it's likely that Nardole was not being serious about this. (TV: The Lie of the Land)