Trickster's Choice by Tamora Pierce

Aly: a slave with the talents of a master spy, a fabled lineage she must conceal, and the dubious blessing of a trickster god.

Sarai: a passionate, charming teenage noblewoman who, according to prophecy, will bring an end to a cruel dynasty.

Dove: the younger sister of Sarai; she has a calculating mind and hidden depths that have yet to be plumbed.

Nawat: a magical young man with a strangely innocent outlook and an even stranger past; Aly’s one true friend in a world where trust can cost you your life.