Kithriarch was a title in Time Lord society given to the head of each of the families of the Great Houses. The Kithriarch was not the same as the Housekeeper, who was responsible for the well-being of the House itself. The Kithriarch was responsible for the social well-being of each family member.
According to Marc Platt Kithriarchs were viewed as father-figures amongst the Household. (Author's Notes: Lungbarrow )