Millennia

Millennia was the second female member of the Deca, and the lover of Rallon, despite their age difference. They attended Prydonian Academy at the same time as the First youthful Doctor.

She was from the House of Brightshore, known for its wealth and power rather than the intellectual achievements of its cousins. However, Millennia possessed 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 the Doctor.

Millennia and Rallon joined the Doctor when he stole a TARDIS and traveled 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 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 Doctor destroyed his realm, releasing them into death. (PDA: Divided Loyalties)