The Insurrectionist Leader, also known as "Dr. Pill" due to the insignia on his armor, was a significant figure among the Insurrectionists, the leader of both the group at the Scrapyard and at the Longshore Shipyards.
The Insurrectionist Leader is a skilled fighter, even without the support of others. He is extremely dedicated to hunting down alien artifacts. Initially, he showed great concern for his fellow members of the Insurrection, but after CT's death, he appears more brutal, more concerned with accomplishing his goals than preserving lives.
It is unknown when CT and the Insurrectionist Leader first met, but he was her contact in the Insurrection and the two had a close relationship. She fed information to him from Project Freelancer about alien artifacts, but after the battle at the Scrapyard, he encouraged her to defect completely, bringing her later to the Longshore Shipyards, despite the distrust of other Insurrectionist members, such as the Blonde Insurrectionist. When that base fell under attack by the Freelancers looking to retrieve CT's armor, he refused to leave the other Insurrectionists and evacuate with CT, instead wanting to stay and fight. In the ensuing battle, he is unharmed, but CT was fatally wounded by Texas.
After CT's death, the Insurrectionist Leader took her armor and codename and continued to pursue the artifacts CT had brought him information about. He ultimately traveled with a group of alien allies to the Temple in the desert, believing it held an ancient powerful energy weapon. They killed most of the legitimate UNSC dig team, but failed to kill Tucker, who sealed himself inside the temple. While they tried to break into it, Sarge, Caboose, and Grif arrived. Initially, the Leader and his allies posed as the dig team, with the Leader calling himself "CT". However, as he began to suspect they were truly there to investigate, Tucker attacked his team and they escaped into the temple.
The Leader and his team continued to try to break into the temple, finally succeeding. However, when the aliens see Epsilon, who Caboose transferred to the alien relic, they stop fighting and begin to worship Church. One of the Leader's men knocks Church out, angering the aliens, who attack the humans, including the Leader. The Leader absconds with Church, but his vehicle is destroyed by Tucker. Finally, as the Leader prepares to kill Tucker, Church reawakens and, angered by the Leader calling him a "thing", obliterates him with his laser face.