According to Temmosus, friendship grew with time. Those bonded by friendship were known as friends. (TV: The Daleks) Charley Pollard and the Eighth Doctor expressed their love for each other as friends. (AUDIO: Neverland)
At various points, the Doctor counted Sarah Jane Smith, (TV: The Seeds of Doom, The Masque of Mandragora) K9, (TV: Underworld, The Invasion of Time, The Pirate Planet) Bernice Summerfield, (AUDIO: The Unknown) Alistair Gordon Lethbridge-Stewart, (AUDIO: Minuet in Hell) Charley Pollard, (AUDIO: Embrace the Darkness) Donna Noble, (TV: The End of Time) Clara Oswald, (TV: Heaven Sent) Graham O'Brien, Ryan Sinclair, and Yasmin Khan (TV: Resolution) among their best friends.