Isaac Fowler is a student at the College, a prestigious institution for magicians, and he first appears during the College Arc as a classmate of Chise Hatori. He is a small-framed young man whose most defining physical characteristic is the hood he wears at all times, deeply pulled down to obscure his features. This persistent concealment hints at a facial deformity he suffered as a child, a vulnerability he guards against the world. His age is estimated to be around fifteen.

His personality is initially perceived as shy, but he considers himself more approachable than some of his peers precisely because he is not a member of the College's elite Seven Shields. Despite his reserved exterior, Isaac is highly knowledgeable and possesses a strong sense of curiosity. This inquisitive nature often leads him to investigate the mysteries surrounding his new friends, and he is frequently the one to provide explanations about the complex magical world to Chise, taking on a guiding role. He has a pragmatic and sometimes surprisingly candid manner of speaking; upon learning about the dark curses and tragic pasts of his companions, he admits to feeling a sense of envy that he lacks such a dramatic backstory, revealing an insecurity about his own perceived ordinariness. However, this self-perceived normalcy masks deep-seated family issues.

Isaac hails from a family of artisans who specialize in crafting weapons and equipment designed to combat magical creatures. As the eldest son, he was originally in line to be the family heir, but he possesses very little natural talent for the family craft, especially when compared to his younger sister who excels. Consequently, he was passed over and disinherited, a fact that has driven him to attend the College since primary school. This history of familial rejection fuels his insecurities and his sharp defensiveness, especially if someone attempts to remove his hood.

Isaac's most significant relationship is with Rian Scrimgeour. They have been close friends since childhood and grew up together, eventually becoming roommates at the College. Their bond is so strong that Isaac has entered into a contract to be the exclusive supplier and crafter of equipment for Rian, a testament to their trust and partnership. Within the wider group of students, Isaac becomes a loyal and vital ally. He is present when the secret of their friend Zoe Ivey being a Gorgon is discovered, and he helps form a blood contract with Chise, Rian, Lucy Webster, and Philomela Sargant to protect that secret. He also develops a protective camaraderie with Philomela, going so far as to temporarily create artificial fingers for her when she loses the ability to perform delicate tasks.

His role in the story is that of a supportive friend and a source of practical knowledge. He uses his intellect to help the group understand the threats they face and offers his magical power to support them in critical moments. During the climax of the College Arc, Isaac is instrumental in the final battle, lending his magical energy alongside Chise and Lucy to empower Philomela to confront her grandmother. After the battle proves successful, he is shown as part of the expanded circle of friends who spend winter break at Chise and Elias Ainsworth's home, participating in festive activities and finally getting to act like a carefree teenager.

As for notable abilities, Isaac's skills lie not in combat but in crafting. He is a skilled magical artisan, capable of creating enchanted weapons and items. This is a reflection of his family's legacy. However, an ironic twist defines his talents: while he has great skill in the forging and creation of magical armaments, he has very little actual ability when it comes to using them. His power is one of support and creation rather than direct confrontation, a fact that aligns with his role as the strategic and knowledgeable heart of his friend group.