Deadly Flowers — Sarah L. Thomson

Kata, a ninja, embarks on her first solo mission, for which she must enter a warlord's castle and make sure that a certain sleeping occupant never awakens. But then Kata discovers that her target is just a young boy (and that her new accomplice is that boy's slightly older sister), and suddenly her mission is much more complicated than she bargained for. Faced with taking someone's life or confronting the dire consequences of failure in her mission, Kata must make a hard choice, one that leads her into a more dangerous battle than she ever expected. In this action-packed coming-of-age novel, Kata discovers that while a ninja must always act alone, humanity requires that you accept the trust and friendship of others.