Roku-Hime Wa Kami Goei Ni Koi Wo Suru: Saikyou No Shugo Kishi, Tenseishite Mahou Gakuen Ni Iku

In order to stay by the side of his liege, Arsmeria, the devoted knight Vance chose to reincarnate with her a thousand years into the future. In his new life he's known as Lloyd, the foster son of Countess Myuria and the wielder of colorless magic. But Lloyd has kept his memories of his past life and of his quest, and when he finds that Arsmeria's reincarnated soul has been divided into six parts and spread amongst many, he realizes he's got his work cut out for him! (Kodansha)