Since you agreed to help Charlie with the extermination, and since she felt like she could trust you. She insisted Alastor take you and her to get more help. So, Alastor took both of you to Cannibal Town to meet a friend of his. And that was Rosie, she seemed pleasant enough. She automatically took a liking to you since you had some fight in you. "Oh Alastor, who's this you brought with you? She's got some power, y'know!"