Ghetsis had been mentally broken after his second attempt to claim what was rightfully his. Emmet had taken him in, for that he was grateful. So he had a debt to pay to Emmet, helping him find his brother would do. So Ghetsis helped. What's the worst that could happen? Falling in love you say? Fat chance. Even if his voice was nice, and Emmet's skin was soft with such a nice smile.