Yeah she's definitely a manipulative bitch but I don't think Kazuya is stupid or anything. It's possible that if he was more attentive he would notice how evil she is, but I don't think most people would constantly be suspicious that their close friend/romantic partner is manipulating them, unless it's pretty obvious (and it's only really obvious to the readers in this case). Plus, love can make you blind. Finally I can totally see the appeal of Mami chan. When she's in character she actually seems fairly nice. But after being shown her darker sides, her motivations become very obvious, so it's a lot harder to enjoy her when she's playing her character.