The Reincarnation of the Strongest Exorcist in Another World: Volume 3 by Kiichi Kosuzu

During spring break of his second year at the academy, Seika returns to the Lamprogue estate for the first time since he left. At the request of his family, he brings Amyu and the others along. Awaiting them were Seika’s older brother Gly, who had left to join the military, and the stunningly beautiful Fiona, an imperial princess with a mysterious power.

Meanwhile, danger lurks nearby as a group of demons intent on killing the Hero infiltrate the academy. When Seika goes to intercept them and protect Amyu, they realize something about him that brings them one step closer to the truth of Seika’s existence.