Angelfire by Courtney Allison Moulton

When seventeen-year-old Ellie starts seeing reapers - monstrous creatures who devour humans and send their souls to Hell - she finds herself on the front lines of a supernatural war between archangels and the Fallen and faced with the possible destruction of her soul.
A mysterious boy named Will reveals she is the reincarnation of an ancient warrior, the only one capable of wielding swords of angelfire to fight the reapers, and he is an immortal sworn to protect her in battle. Now that Ellie's powers have been awakened, a powerful reaper called Bastian has come forward to challenge her. He has employed a fierce assassin to eliminate her - an assassin who has already killed her once.
While balancing her dwindling social life and reaper-hunting duties, she and Will discover Bastian is searching for a dormant creature believed to be a true soul reaper. Bastian plans to use this weapon to ignite the End of Days and to destroy Ellie's soul, ending her rebirth cycle forever. Now, she must face an army of Bastian's most frightening reapers, prevent the soul reaper from consuming her soul, and uncover the secrets of her past lives - including truths that may be too frightening to remember.

A very enjoyable story.

What I particularly liked was how Ellie has this huge amount of power available to her, but it's being limited for reasons she doesn't know. Will, her guardian, is there to protect her even if it means giving up his life, and he's done a great job of keeping Ellie alive even in her weakened state. However, Ellie is no slouch herself. She can kick butt with the best of them, even when it's close to driving her insane.

The romance aspect was very well done. It slowly built up, and Ellie develops feelings for her love interest as she gets to know him--as we get to know him. But I especially liked how Ellie refused to lead another boy on simply because he was interested in her. Very refreshing.

Actually, I liked all the characters, even the ones with questionable behavior. I'm thinking that these strange quirks will have explanations later on. At least, I hope so. Some of it is too extreme not to have an outside influence, and I'm curious to find out what that could be.

The only drawback was the slight predictability at the end. When we find out where they are going and how they are getting there, we all know what's coming. Fortunately, Moulton makes up for it with a surprising reveal after the climax. It adds a whole new level to everything, especially her romance. I'm very much looking forward to the next book.