Phoenix of the Sea by Madalyn Rae

Everyone dies, or at least they do in Adria Kane’s world. Growing up in the L.A. foster care system, she never had a stable home or knew what real love was. Given the choice of prison or military, Adria joined an elite group of soldiers and became a Phoenix—humans trained to fight Elementals and the horrors they’ve inflicted on the earth and humankind for eternity.
When Murphy McKenzie, a Phoenix from the UK, arrives to train with Adria, her world implodes. Not just because he’s six and a half feet of distraction, but because the secrets he holds of Elementals and who she truly is, threaten to change her life forever.
As tensions rise and secrets are revealed, Adria must make a choice that will give her everything she’s lived her life without. Thrust into a world of hybrids, castles, money, and love, she’s forced to question the life she thought to be true. To save Murphy’s life and the fate of the planet, she must choose to fight it or accept the power she was born to hold.