Oswald Grimsarde is a student at the royal academy in Villainess Level 99 and one of the capture targets, or romance options, of the otome game Light Magic and the Hero on which the world is based. He comes from a high-ranking noble family, being the son of a kingdom minister, and is widely regarded as a magic prodigy. He is a teenage boy with blue hair reaching chest length and blue eyes, and he is usually seen wearing glasses. His demeanor is cool and composed, and he can appear cold or aloof, although he possesses a quietly kind nature; for instance, he discreetly helps Alicia Ehnleit when she is in trouble.

As a mage, Oswald is exceptionally gifted, with an unmatched ability to use all four classical elements, and his skill in this area surpasses that of Prince Edwin Valschein, another of the game's love interests. His pride in his abilities is central to his personality, and that pride is shaken when Yumiella Dolkness destroys a practice target dummy that he had only managed to dent. The incident gives him an inferiority complex and leads him to swear revenge against her. He initially behaves arrogantly and dismissively toward Yumiella, but witnessing the full extent of her power forces him to change his view of her. As the story progresses, his resentment toward her gradually fades.

In the story, Oswald serves as a supporting figure within the academy, interacting with the main cast, including the heroine Alicia, Prince Edwin, and fellow student William Ares. His relationship with Alicia is shaped by his role as one of her potential love interests, and his hidden kindness is shown through the way he looks out for her. His relationship with Yumiella is the most significant to his development, as it challenges his sense of self and moves him from rivalry toward acceptance. Through this arc, he develops from a proud young noble defined by his status as a genius mage into a more grounded person capable of recognizing strength in others.