Mithias lifted his eyes toward the girl, and her battle-ready company. He got it. She was in charge here and had the situation under her control. She could have shot him and destroyed him to be rid of him, as far as Mithias knew, but she did not. The other souls here were very powerful and not to be taken on alone. The slyness of his master flowing in his veins, Mithias put it together and bought into the ruse. 

Opening his arms and showing his shackles, he hoped to show that he had been held against his will. "Please, do not destroy me! I am merely a slave. I was once human. I have paid the price for my misdeeds. I... see you are strong, and you can fight hell's forces. Let me join you." Mithias was offering to make himself a tool for Mina's use. The others would have no idea that he intended any betrayal.