Sheep (羊, Hitsuji?) is a group consisting largely of teenagers that maintain their own territory and begin attacking the Port Mafia and other organizations. While the group goes by this name, it has no official name.
The Sheep protect their territory from any possible threats. Sometime after "forming", Chūya joined the Sheep. 
As a result of Chūya Nakahara's reputation, the group rarely deals with enemies, as any potential enemies are scared off by Chūya's power. As the group focuses on defending its members and territory, they're well-known for defense alongside Chūya's prowess in battle. Consequently, the Sheep depend heavily on Chūya and many claim Sheep cannot survive without him.
After the Sheep dismiss Chūya from their ranks, they join forces with the Gelhart Security Service - a marine-based force that attacks ships - for protection. After the Port Mafia slaughtered GSS, they refrained from attacking the Sheep.
The Sheep consists of delinquent, down on their luck youths, that formed a support chain and their own territory to protect one another. Although young, they developed a deep understanding of dependency on one another, especially their leader.
This leads to their 'Council', a group of thirteen members that must cooperate with each other in order to make decisions for Sheep. As a result of the Council, it is considered unethical for a single Council member to call the shots on when a new member is inducted without further approval and discussion. Thus, while one person may be considered "leader", there are twelve others the leader must work with to maintain harmony in the group.