One way to go about it would be to build Cecilia's character now and work backwards. In other words, decide on her personality, likes, dislikes, skills, that sort of thing, and then work out how she got there. For example, say that she has arachnophobia; did she find a really really big spider in closet? Or maybe she got bit at one point and had a really bad reaction to it? The most important thing is to make it interesting.