Bring Me Their Hearts, by Sara Wolf
Zera is a Heartless―the immortal, unaging soldier of a witch.
Bound to the witch Nightsinger, Zera longs for freedom from the woods they hide in. With her heart in a jar under Nightsinger's control, she serves the witch unquestioningly.
Until Nightsinger asks Zera for a prince's heart in exchange for her own, with one addendum: if she's discovered infiltrating the court, Nightsinger will destroy Zera's heart rather than see her tortured by the witch-hating nobles.
Crown Prince Lucien d'Malvane hates the royal court as much as it loves him―every tutor too afraid to correct him and every girl jockeying for a place at his darkly handsome side. No one can challenge him―until the arrival of Lady Zera. She's inelegant, smart-mouthed, carefree, and out for his blood. The prince's honor has him quickly aiming for her throat.
So begins a game of cat and mouse between a girl with nothing to lose and a boy who has it all.
Winner takes the loser's heart.
"Zera is an exemplar of confident, capable, relentlessly witty young womanhood. She is assertive, proactive, and, above all, a fun character to follow, yet she's sufficiently well-rounded. ... A zesty treat for YA and new-adult fantasists." –Kirkus Reviews