Isaac is a young man with fair skin, blue eyes, and blond hair. His typical attire consists of a white sleeveless jacket left open, dark blue trousers, black boots, and black wristbands. On occasion, he has also been seen wearing a dark brown warm-up jacket with white trim around the neck.

Isaac’s personality is defined by a central contradiction. He presents himself as arrogant and presumptuous, trying to appear brave and tough. However, beneath this facade, he is often cowardly and timid, especially when faced with real danger. He is deeply and stubbornly concerned about a woman named Cindy, for whom he has romantic feelings. His primary motivation throughout the story is to prove that Dante is a dangerous individual, as he believes Cindy’s admiration for the demon hunter puts her at risk. While persistent in his efforts, Isaac can become discouraged and give up when things do not go according to plan. Despite his flaws, he has shown a capacity for self-improvement, setting aside his pride to emulate the qualities that Cindy admires in Dante.

In the story, Isaac first appears at a restaurant called Fredi, where he works up the courage to ask Cindy for a date. Cindy refuses him, explaining that she prefers a man who is elegant, wild, handsome, kind, and trustworthy—qualities she sees in Dante, and she suggests that Isaac might have a chance if he were more like him. Taking this as a challenge, Isaac begins to follow Dante, hoping to uncover his true nature. His amateurish stalking leads him into a series of mishaps. He follows Dante to a club, where he witnesses Dante enter a restroom with a woman (who is later revealed to be a demon) and is subsequently thrown out by a security guard. On another occasion, he spies on Dante’s office but is discovered by Patty. Desperate for proof, Isaac breaks into Dante’s business, where he is frightened by demonic specters. There, he encounters a demon named Sid, who gives him a small, silver, skull-shaped bell, claiming it will help Isaac discover Dante’s true identity. The bell, however, turns out to be a charm that summons demons. After a demon summoned by the bell attacks Isaac, Dante arrives and kills it. Isaac’s final attempt to flee the city ends with his car being destroyed during a large-scale battle in which Dante uses the bell to summon and destroy a powerful demon army. In the end, Dante returns a photograph of Isaac and Cindy, encouraging him to talk to her before leaving. Isaac takes this advice, later joining Dante at the restaurant to share the same dessert, with Dante interpreting this as an unwanted display of admiration.

Isaac’s key relationships drive the plot of the anime’s early episodes. His central dynamic is with Cindy, the object of his affection, whose rejection and comparison of him to Dante spark his entire mission. His antagonistic relationship with Dante is not driven by malice but by misguided jealousy and a desire to protect Cindy from someone he believes is a threat. He acts as a foil to Dante, representing the viewpoint of an ordinary, insecure human reacting to the extraordinary world of the demon hunter. He also has brief but significant interactions with other characters, including being chased off by Patty and being manipulated by Sid.

Isaac undergoes a notable but subtle development. He starts as a prideful man trying to win a woman’s affection through deception and proving a rival unworthy. After a series of humiliating experiences—being thrown out of a club, falling off a second-floor window while fleeing, and having his car destroyed—he finally accepts reality. Dante’s final act of returning his photograph and telling him to speak with Cindy serves as a catalyst for Isaac to abandon his jealous schemes. He concludes his arc by choosing to be more like Dante in the one way that matters: with actions rather than words, as shown when he sits down to eat with his former rival, signaling a path toward humility and perhaps even friendship.

Isaac has no supernatural abilities or special combat skills. His notable ability is persistence in his stalking, despite repeated failures and physical injuries, such as falling from a window and breaking his foot. As a normal human, he possesses a revolver, which he is prepared to use, but his fearful and clumsy nature prevents him from being an effective adversary. His role in the story’s action is observational; he is a witness to Dante’s power rather than a participant in it.