Champion of Fate by Kendare Blake

#1 New York Times bestselling author Kendare Blake is back with an epic duology starter that follows a young woman training to join a fabled order as she attempts to lead a hero to his critical first victory. A must-read for fans of Alexandra Bracken and Victoria Aveyard.
Aristene are an order of mythical female warriors. Though heroes might be immortalized in legends, it's the Aristene who guide their paths to victory. They are the Heromakers.
Raised by the order after being orphaned, Reed grew up surrounded by her future sisters-in-arms and the incredible stories of their quests. She's been counting the days until her initiation, and now one final test stands in her way: shepherding her first hero to glory on the battlefield. Succeed, and her place in the order is secured. Fail, and she'll be cast out of the only home she's ever known.
But Reed didn't count on Hestion, her assigned hero, being both infuriating and intriguing. When their strategic alliance turns into something more, it forces Reed to question the cost of becoming an Aristene. As battle looms and fate hangs in the balance, Reed must make an impossible choice: her hero or her order.