Champion of Fate by Kendare Blake

The Aristene are an elusive order of immortal female warriors. To be championed by them is to become a legend. They are the Heromakers.

As an orphan raised by the order, Reed has always envisioned taking her place beside her sisters-in-arms. To do that, she'll have to help her first hero accomplish a great victory. But what happens when her charmingly infuriating hero makes her question everything she's been taught about being an Aristene and the order itself.