Millie Peterson was a young girl who was the cousin of Louie Rollins and the daughter of Rachael Peterson. She aided the Eleventh Doctor on several occasions.
Millie and Louie had been to an enjoyable Halloween party the night before Louie encountered the Doctor in a warehouse. Louie, Millie and the Doctor were enjoying Bonfire Night. The effigies of Guy Fawkes sprang to life, a result of an alien that had been staking out the warehouse. The Doctor took Millie and Louie with him in the TARDIS to the warehouse. They refused to stay in the TARDIS while he dealt with the alien. After the alien and its equipment had been destroyed, they returned to Bonfire Night to enjoy toffee apples. (PROSE: The Night After Hallowe'en)
At Christmas, Millie called the Doctor and invited him to join her mother, Louie and her at a pantomime of Peter Pan. Though the Doctor was busy, he later remembered the call and arrived at the theatre. The theatre's power was sabotaged by a Brakari,who needed the energy for her baby and her. When Louie became cold and weak, Millie and the Doctor retrieved blankets for him. On the way back, Millie tripped on the icy carpet and almost fell off the balcony, but the Doctor caught her. The baby Brakari attached herself to Millie's back for energy, though the Doctor sonicked it off. She later helped the Doctor reach his TARDIS, which was in a deadlock sealed room, by realising that he could use the sonic screwdriver on the screws of the door's hinges. After helping the Doctor carry equipment to help the Brakari and Louie, she joined her mother, Louie, Big Jack and him in the TARDIS to release the Brakari in space. (PROSE: Snowfall)
The next year, there was a snowstorm and widespread blackout caused by the Council of Dead. When Louie was visiting her, Millie was phoned by the Doctor. He had been pulled off course by the Council and was trying to figure it out. Millie pointed out the snow was still fluffy, even though it should have melted and refrozen, allowing the Doctor to identify the phosidium in it. While the Doctor, Louie and Rachael went to the TARDIS to search for more, Millie investigated a snowman who seemed to have glowing eyes. When she touched it, it came to life, attacked her and teleported her to a factory across town. She was strapped to a table by Fasheith, who planned to use her to attract the Doctor. She freed herself by lubricating her wrists with chapstick and escaped. She encountered the Doctor on the way to save her. He gave her instructions to the TARDIS, a homing device to pilot it to him and another device to destroy the council. This device destroyed the Council by making them lose control of their bodies, making them decay into dust. After these adventures, the Doctor took Millie and Louie to see the birth of the Salcreyan Nebula. (PROSE: Attack of the Snowmen)
Appearance
Millie was shorter than Louie, with curly black hair. (PROSE: The Night After Hallowe'en)