4. Reinette (Madame Du Pompadour)
Joan Redfern wasn't the only character on this list whom the Doctor fell for. In The Girl in the Fireplace he met French historical figure Madame Du Pompadour, or Reinette as she initially introduces herself. The Doctor meets her at various points accross her timeline, investigating what the Clockwork Droids want with her. However, their relationship develops with the Doctor filling the role of Reinette's imaginary friend and appearing whenever she needs him. She comes to mean a lot to Ten; he even risks being trapped in history for centuries to save her from the Droids' endgame. With the robots out of the way, he offers her the chance to take a trip in the TARDIS (which knowing the Doctor, would probably have escalated into more trips). However, on returning to check whether she's ready to go, he discovers that she's died in his absence. She's left him a letter which, from the look on his face after he reads it, affects him deeply, giving the episode a bittersweet ending.