The Girl (Guardians, #1)

Emmy - a bookish, witty student - is rescued by six teenaged angels with powers, called Guardians. They've been sent to elicit the secret from her before the demons can. Emmy later learns that her name is the only clue to locating a sought-after bridge that for centuries, Evil has plotted to destroy; this would in turn bring about the end of humanity.

Things get worse when Emmy falls madly in love with Marcus, the well-intentioned but obstinate leader of the Guardians. He eventually confesses that he is in love with her as well. Unfortunately, Marcus has been warned by his clairvoyant and omniscient guide that should he and Emmy get together, the mission will fail and humanity will perish.