Guardian by Kerri Morrigan

Kalev has been working for an eternity to prove himself worthy of readmittance into heaven after God cast out any angel that still had lessons to learn about mankind. Since then, he has done everything right. With one exception – he saved a little girl he was ordered to let die. Twenty years later, when the Celestial Council presents him with an opportunity for promotion, Kalev knows that if he squanders this chance he may never get another. When he is assigned to guard the woman who caused his downfall in the first place, he must choose between obeying the Council or once again keeping the promise that had already caused him so much trouble.
After aging out of the foster care system, trust is no longer a word in Grace Lightbourne’s vocabulary. Tired of lies and disappointment, she now devotes her life to helping children in foster care find safe, loving homes like she never had. When Grace is attacked by a gruesome creature in the park, she discovers a realm of demons unknown to her, and she has no choice but to accept the help of the handsome stranger, Kalev, who intercedes and pledges to protect her. But for Grace, trusting Kalev means choosing to be vulnerable, and the longer she stays, the harder it will be to keep her heart closed.
Kalev knows he is all that stands between a Marquis demon who has designs to enslave Grace and, through her ancestral ring, create an army of souls to conquer the world of the living. But what at first is just a mission becomes the most difficult choice of Kalev’s existence—deny his feelings and follow the rules of the Council or sacrifice his place in eternity for the woman he has grown to love.