Mika. The Ember Town Series, #4

Mika was raised to believe that magick is binary-and so is love. Adopted as an infant for her extraordinary power, Mika has always known it was her destiny to become the head of Ember's coven. However, destiny comes with conditions: the elders have decreed she must marry her best friend, Rob, before Samhain. Mika has no interest in the life they've planned for her. Instead, her heart is drawn to Birk, a non-binary nymph she hasn't even had the chance to take out on a date.
As Mika defies the elders' plans, she begins to uncover secrets about magick that challenge everything she was taught. With Samhain fast approaching, she must decide whether to follow the path laid out for her or carve out her own.