Moon Dark Smile by Tessa Gratton

The fate of an Empire lies with a headstrong Heir and a restless demon in this lush YA fantasy for fans of Laini Taylor and Girl, Serpent, Thorn.
Ever since she was a girl, Raliel Dark-Smile's best friend has been the great demon that lives in the palace. As the daughter of the Emperor, Raliel appears cold and distant to those around her, but what no one understands is that she and the great demon, Moon, have a close and unbreakable bond and are together at all times. Moon is bound to the Emperor and his two consorts, Raliel's parents, and when Raliel comes of age, she will be bound to Moon as well, constrained to live in the Palace for the rest of her days.

Raliel is desperate to see the Empire Between Five Mountains, and she feels a deep kinship with Moon, who longs to break free of its bonds. When the time finally arrives for Raliel's coming of age journey, she discovers a dangerous way to take Moon with her, even as she hides this truth from her travel...