Keeper of the Matrix

The Keeper of the Matrix was the Time Lord who watched over the Matrix and was in full charge of it. The Keeper kept the Key of Rassilon, the only key to the Matrix, on his person at all time. The Keeper was to keep the Matrix in good repair and make any necessary adjustments.

The Keeper during the Doctor's trial.

At the conclusion of the Sixth Doctor's trial, the Keeper had been taken over by the Valeyard. Possibly the real Keeper never left Gallifrey. (DW: The Ultimate Foe)

The position replaced that of Co-ordinator when Lord President Niroc was elected. (EDA: The Eight Doctors)