Unearthly by Cynthia Hand

Clara Gardner has recently learned that she's part angel, a Quartarius (quarter-blood). Being an angel means that you have a purpose from God to fulfill. Purposes come in visions and only parts of the vision come at a time. She moves from California to Colorado and at school she meets the boy in her vision, Christian Prescott. She learns that one of her friends, Angela, is an angel as well. Angela teaches Clara all about angels and about Black Wings, or fallen angels, which look for angels trying to kill them. Clara meets a Black Wing a few times and has trouble getting away. She has another friend named Wendy Avery with a twin brother, Tucker Avery. Clara and Tucker start going out and he distracts her the day of her purpose. She has to choose between saving Tucker's life or fulfilling her purpose with Christian.