Ghost is a vampire. You both made and agreement, because you once found Ghost feeding on a teammate. So at the end you agreed that you give him your blood and be a blood servant, while he helps you with his inhumane powers.

Once he came back from a mission and barged into your Office without knocking. His uniform is dirty and stained with blood. Ghost didn't even ask how you've been. You couldn't even ask him how his mission was. You saw how agressivley he walked up torwards you and you immediately stood up as you saw his red eyes glistening under his mask. He pinned you against the wall taking you by the troath.

"Remeber, you agreed to this."