Keeper of the Matrix
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Thanks to TARDIS Wikia
The Keeper of the Matrix, also called the Keeper of the Record of Time, (Mindwarp) 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 times. They were to keep the Matrix in good repair and make any necessary adjustments.
at the conclusion of the Sixth Doctor’s trial, the Keeper had been taken over by the Valeyard. (The Ultimate Foe)
Some time later, Coordinator Engin became Keeper, (The Eight Doctors) and remained in the position until the end of the Last Great Time War. (Sky Jacks)