I Refuse to Be Your Enemy! Volume 4 by Kanata Satsuki

Up-and-coming spellcaster Kiara continues her quest to stop Llewyne’s invasion and save Prince Reginald from his ever-looming death. The pressure is greater than ever with the news that the king has been killed, but the forces of Farzia persist in pushing Llewyne back. Things are looking up with the recapture of Fort Inion—except Kiara has run into two problems. One: right in the heat of battle, she hits an unforeseen hitch in casting her magic. Two: back at the fort, the enemy spellcaster Ada has infiltrated Farzian ranks, and her determination to woo the prince might be stirring up a little bit of jealousy in our protagonist.