I'm a Dark Knight but Aim to Be the Strongest Holy Knight

Story:

Kai, who comes from a family that specialized in dark-style occupations, sought even greater strength. One day, he saved the fourth royal princess who was under attack by monsters. This was the the trigger for him managing to join an academy that specializes in training people for holy occupations. He endures training so incompatible with his physical constitution that it could make him faint, all to become the polar opposite of the strongest dark knight—the Kingdom's strongest holy royal knight, The Royal Paladin. An occupation-change drama of a genuine dark knight begins!!