She was from the House of Brightshore, known for its wealth and power rather than the intellectual achievements of its cousins. Millennia had a natural gift for temporal engineering, and the Doctor was certain she would prove to be an exception. Her blue hair and gentle nature were her most distinctive traits, according to him.
Millennia and Rallon joined the Doctor when he stole a TARDIS and travelled to the realm of the Celestial Toymaker. The trip ended in disaster, with Rallon absorbed as the Toymaker's new host body, and Millennia trapped forever as one of his living dolls.
On returning to Gallifrey alone, the Doctor was put on trial and expelled from the Academy, and Millennia and Rallon's names were erased from Time Lord history. They remained with the Toymaker until the Fifth Doctor destroyed his realm, releasing them into death. (PROSE: Divided Loyalties)