Fleabee's Fortune is the first book in the Deptford Mouselets Series by Robin Jarvis (first published in 2004).
The story, taking place in the year before the events of The Dark Portal, is about a young ratgirl named Fleabee, who is unlike the rest of her kind. She is gentle and does not want to harm anything. But the festival of the First Blood is approaching, when young ratlings must murder by midnight or be killed themselves. And so Fleabee has to make a difficult choice.