Book One of The Eternity Series by S.J. Garrett.
Ghost Eyes (Eternity Book 1)
Life. Death. It's all the same to Gardina Campbell, the mysterious Gray Etyrnal. When the immortal Ghost Hunter meets Commander Matthias Logan as he abandons a war he can't stand, her existence changes for all time. Their Devotion might just be the only thing that can save their countries--if Gardina can keep them both alive long enough to see the end.