Mizukage

The is the leader of Kirigakure. There have been six official Mizukage and their word holds sway over all political and military issues that present themselves regarding Kirigakure and its inhabitants. The Mizukage's building is the widest and largest landmark in Kirigakure, bearing the kanji for on it.

Fourth Mizukage
He was manipulated by Tobi who could be called the "real" Fourth Mizukage.

Trivia

 * Unlike the other Five Kages where there is some sort of connection between predecessor and successor, the Mizukage position seems to filled by random shinobi who are simply powerful enough to attain the title with little connection to their predecessors.
 * The Third was the bodyguard for the First.
 * The Sixth was the bodyguard for the Fifth.