Kougekiryoku Zero kara Hajimeru Kenseitan: Osananajimi no Koujo ni Suterare Mahou Gakuen ni Nyuugaku shitara, Maou to Keiyaku suru Koto ni Natta

Eugene Santafi eld is a young man who dreams of becoming a magic swordsman. There's only one problem: He can't use any attack magic! Instead, he's blessed with extremely powerful defensive and healing magic, an irony that earns him the ridicule of his peers. Determined to escape from those who know of his failings, Eugene flees to the Lykeion Magic Academy where he meets Erinyes, a fallen Demon Queen imprisoned beneath the school. Taken in by his potential, Erinyes appoints Eugene to be her caretaker. Not long after, the headmaster of the academy assigns Eugene to be the guardian of Sumire, a mysterious girl from another world who just so happens to be an Ifrit of legend. With these two women at his side, history's biggest failure is sure to become a legendary swordsman...right? (Source: J-Novel Club)