In The Lord of the Rings, the wizard who becomes the head of the White Council is Saruman.
Saruman the White is the chief of the Istari, the order of wizards sent to Middle-earth to oppose Sauron. He is also the head of the White Council, an alliance of powerful figures including Galadriel, Elrond, Gandalf, and Círdan. The council is formed to coordinate resistance to Sauron and investigate the growing threat in Middle-earth.
Saruman’s position gives him great authority, but his desire for knowledge and power gradually corrupts him. He studies the methods of Sauron, builds the fortress of Isengard, and eventually seeks the One Ring for himself. His betrayal becomes clear when he imprisons Gandalf at Orthanc and later uses an army of Orcs and Men against Rohan.
Gandalf is another member of the Istari, but he does not become head of the White Council. Alatar and Radagast are also associated with the wizards, yet neither holds Saruman’s leadership role. Saruman is called “the White” until Gandalf defeats him and takes that title.