Devero leans right back against Kaiba, and makes the decision in that moment not to reveal himself to the younger man. Not right now. He'll confess later, when they're through all of this. Kaiba needs the dog.

So he lets his plumed tail wave back and forth with more vigor after Kaiba's words, and lets out a single, huffy boof to punctuate him. Positive is easy for a dog; he positively radiates the trust and confidence in the human at his side.