Between Realms by Lexi Ostrow

Astrid's forgotten past was within her reach. Robin's greatest mission is to protect a princess who doesn't belong. They'll have to reconcile in order to keep peace within the Fae Realm .

Astrid went to sleep after her father's death in Ireland and woke up in New York City. She's never felt she belonged in her new life, but her family refused to speak about "before". High school graduation brings her freedom, and with it, the chance to return to Ireland and try to put together the pieces she can't remember. Until an actual unicorn appears in her kitchen and she has no desire to remember anything.

Ronin would give his life for the king, but masquerading as a human to protect his daughter is almost too much. Astrid isn't safe in their realm thanks to her human blood, and Ronin wants to keep her far from the Realm. Until he realizes Astrid may not be entirely human after all, but part unicorn after all. The more time he spends with her, the more he wishes...