Rogue. The Valiant Series, #5

Lynia has always dreamed of being a Conclave Mage. To join them, she goes on dangerous jobs with constructs she summons using magical keys. Lynessa is nothing like her sister. All she wants is a normal life, but she keeps getting dragged into Lynia's schemes. It doesn't help that she finds herself attracted to one of the constructs-a being of magic, not flesh. Along the way, Lynia discovers a mysterious black key, one that summons a rogue mage who is powerful enough to disobey her commands.
He also makes her laugh and experience feelings she's never had before. But the rogue mage knows more than he's letting on, and the sisters discover dark secrets that threaten to destroy life as they know it. Together, they must work to save their world... or watch it be destroyed by darkness.