Beyond the Border Forest: Into the Prawdziwy Las (The Immortal Seasons Cycle) by Molly Haniszewski

Two kings rule the ancient woods—the Oak King in summer, the Holly King in winter—and no mortal may cross into their realm without leave. But when eighteen-year-old Rowan Gwózdek’s sister vanishes, Rowan defies the old laws and enters the forbidden forest, desperate to find her. She isn't alone for long. Keziah, a trans (and very handsome) hunter with a shadowy tie to the Oak King, offers his help—and a perilous path to the Bear Maiden, a creature said to grant wishes. Together, Rowan and Keziah face mossmen, a wood dragon, and other servants of the forest kings. But danger lurks deeper still. Caught between the warring Oak and Holly Kings, Rowan is forced to take sides in a battle far older and fiercer than she ever imagined. Saving her sister could cost her everything—including her way home.