Yzma is malicious, funny, silly, raspy and power-hungry. However, it seems that she raised Kuzco, rather than his parents, and has been an advisor and the chemist to his family for many years. It is only when Kuzco abruptly and callously fires her that she turns on him, deciding to kill him, then fill the power vacuum left in his wake and become empress. It's also implied that her being the one who raised him is part of the reason for Kuzco's negative personality traits.