Oni no Ou to Chigire - Chapter 2 Pg 59

Takakage who just became a first year university student, smiled with the face of a good-natured older brother. Fujitsugu explained the gist of the ceremony to him, while Kiwa prepared his meals, and helped him change into the ceremonial kimono. Takakage had settled his own contract there four years ago. Since his memory of the ceremony was most fresh, he told Tokimori about his own experience, and eased his tension. During that time, Yato sat on Tokimoris shoulder, sticking close to him, and keeping so quiet it was like all that talking he would normally do was lie. Even though these demon masters should have been able to see Yato, no one asked Tokimori about him at all. A demon master who already made a contract with a demon servant would by no means show any interest in another demon. Demon masters talking about their demons was taboo. The reason was because their own demon servant would get jealous. Whether they were passive, or belligerent, demons each had their own personality, but most demons were immensely possessive, and insanely jealous. If a demon master showed any interested in a demon other than the one contracted to him, they would get violent to show just how possessive they were. If a fight to the death between demons broke out, demon servants have been known to kill their own demon masters and eat them. By doing so, they would literally become one body and soul.