The Council of Thirteen is the ancient ruling body of the Yeerk Empire, which even the highest and most powerful Vissers answer to. One of the council members is actually the Yeerk Emperor, but no one but the Council members know who it is, in order to combat assassination attempts. In order to kill the Emperor, one would have to kill all thirteen. The identity of the Emperor is not revealed in the series, but Visser One implies that her mentor, Garoff, is probably the Emperor in VISSER. A seat on the Council of Thirteen is the highest position in the empire, which those, such as Visser Three, aspire to be in.