Leonhard von Glanzreich, the fourth prince of Glanzreich, possesses the royal family's signature striking blond hair and blue eyes, consistently maintaining a formal and neat appearance. Frequently described as the kingdom's most beautiful boy, an observation noted by his tutor Heine Wittgenstein, his outward demeanor often appears haughty and dismissive of commoners. Beneath this facade, however, lies significant social awkwardness and a tendency toward extreme self-criticism over minor matters. Despite not being the youngest, he exhibits pronounced childish traits: food pickiness (disliking carrots and bell peppers while strongly preferring sachertorte), spontaneous sulking, crying when upset, and persistent jealousy. He frequently gives up on challenges or attempts to escape difficult situations, reflecting an initial lack of resilience. A tsundere aspect manifests as he masks his desire for affection and praise behind aloofness or rudeness. He is notably honest, often making blunt or hurtful remarks unintentionally, and proves an exceptionally poor liar due to his transparent emotions. Heine Wittgenstein once observed Leonhard's lack of practical knowledge—understanding money, eating without utensils, or picnicking without formal setups—would render him incapable of survival outside royalty.

Leonhard's aversion to academics and tutors originates in childhood trauma; his first tutor subjected him to physical abuse, including beatings with a rod, fostering a deep-seated hatred for educators and a pattern of fleeing perceived threats. This abuse contributed to his academic struggles and initial rejection of Heine as his tutor. His development begins with Heine's arrival. Leonhard initially greets Heine with false friendliness before openly rejecting him, consistent with his past behavior. Heine's patient and non-abusive approach gradually earns Leonhard's trust, leading to eventual acceptance. This relationship fosters significant growth: Leonhard becomes more diligent in studies and responsibilities, particularly after his older brother Eins nominates him to succeed their father as king in Chapter 103 of the manga. This nomination catalyzes Leonhard's maturation, as he actively works to overcome his weaknesses and prove himself worthy of the throne. His simplistic perspective evolves from a hindrance into an asset, enabling him to devise straightforward solutions to complex problems, while his strong sense of justice and athletic prowess become more pronounced.

Leonhard maintains a reflective diary documenting daily regrets and negative emotions, serving as an outlet for his self-critical nature and underscoring ongoing internal struggles. His bond with Heine deepens, marked by jealousy upon learning Heine tutors other princes, though Heine reassures him of his unique value. By the series' later stages, Leonhard remains at the palace preparing for kingship while his brothers pursue separate paths, actively demonstrating a commitment to his duties that contrasts sharply with his earlier tendency to flee challenges.