The Keeper of the Dark Brotherhood is a rare and important officeholder in that sinister organization. When there is no Listener designated amongst the membership by the Night Mother, the Brotherhood is then out of contact with Sithis and the Night Mother until a member appears whom she finds worthy. Until then, the unhallowed remains of the Night Mother, kept in a special casket, are protected and accompanied by the Keeper, who vigilantly protects and when necessary transports these remains in order to preserve the basis for the Brotherhood and the Night Mother's ability to select a new Listener once one arises.
There is no apparent communication between the Night Mother and the Keeper as there is in the case of the Listener. Keepers can be quite jealous of this relationship considering the investment they make in the Night Mother as her caretaker, as demonstrated by Cicero's relationship with the Listener she selects in the Fourth Era, in Skyrim.