Jeffrey Stuart is the first prince of the Kingdom of Sorcier and the eldest of the four Stuart brothers, which includes Ian, Geordo, and Alan. He makes his first appearance in the second season of the anime, where he is introduced as a new character who fits seamlessly into the existing narrative.

In terms of appearance, Jeffrey has silver hair and emerald green eyes, a noble look that immediately conveys his royal lineage. His physique shares some similarities with his younger brother Alan. His personality is layered and somewhat deceptive. On the surface, Jeffrey is extremely friendly, sociable, and relaxed, often wearing a constant smile and maintaining an air of distraction. He approaches his days with a playful and even eccentric attitude. However, beneath this laid-back facade lies a sharp and capable individual who takes his princely duties seriously. He is a competent leader who manages state affairs with dedication. One reviewer has noted that Jeffrey, like the protagonist Katarina Claes, is a selfless and helpful person who cannot abandon anyone in need, working hard for the sake of others rather than for his own romantic interests.

Jeffrey's primary motivation is an intense, almost obsessive love for his three younger brothers, whom he considers adorable and the very reason for his life. This adoration manifests in peculiar ways, such as hiding giant portrait panels of his brothers behind a curtain in his office so he can look at them multiple times a day, exclaiming over how cute they are. He also secretly collects discarded personal items belonging to his brothers. His behavior towards Alan, the youngest, is particularly overbearing and physical, often leaving Alan feeling smothered. Despite being the first in line for the throne, Jeffrey has no interest in becoming king. Instead, he has devised a unique strategy to protect his brothers and ensure the kingdom's future. He deliberately gathers opportunistic and harmful nobles around himself, creating a faction of problematic individuals in order to monitor them and orchestrate their downfall, thereby removing any future threats to his siblings and the nation. The apparent rivalry among the brothers for the throne is merely for show as part of this larger plan. Within the story, this strategy positions him as a wildcard who is unpredictable even to his brother Geordo, who holds a certain awe for him.

Regarding key relationships, his most significant bond is with his brothers, though it is one-sided in its most extreme expressions, as only a few people are ever aware of the true depths of his fanatical brotherly love. His other primary relationship is with his official fiancée, Suzanna Randall. Their engagement is not a romantic one but rather functions as a business partnership based on mutual understanding, respect, and common interests. Suzanna is a brilliant magic researcher and the head of the magical artifact laboratory at the Ministry of Magic. She is fully aware of Jeffrey's peculiar and eccentric nature, often calling him a pervert or a weirdo, yet she also respects his abilities. They work together effectively, sharing a desire to protect the kingdom for the sake of his younger brothers, and Jeffrey appreciates her independent genius and allows her to focus on her research without the constraints of court life.

Jeffrey does not possess any flashy magical abilities but is instead defined by his sharp strategic mind and his capabilities in handling state affairs. He also has a great fondness for magical items, frequently commissioning the creation of new ones from his acquaintances at the Ministry of Magic.