The Unholy Paladin

Kai, a young Dark Knight who comes from a household of elite Dark-job holders, decided to pursue even greater strength. One day, he saved the Princess who was being attacked by a monster, which ended up being the trigger for him to join an academy specialized in teaching Holy jobs. Together with Millia, his childhood friend as well as a Paladin, his little sister Chloe the Necromancer, and Princess Shel, who later becomes his tutor, their thrilling academy life begins. Despite having a body so incompatible with Holy magic that he almost passes out whenever he uses it, he is resolved to endure all hardships, for he aspires to become the polar opposite of the strongest Dark Knight: the kingdom's strongest Royal Paladin!

(Source: Manga UP! Global)