He appears friendly, but he is in reality very selfish, hypocritical and two-faced. Like his brothers he is also a do-sadist. Everything that surrounds his ego must always be at his mercy and he uses his pretty face to his advantage whenever he can.  Kou works as an idol in the human world, and is used to hearing his name and seeing his face on the covers of magazines when he passes by on the street. Perhaps that's why the fact that Yui isn't 'as impressed' with him as expected irritates him.