Isaac Stern is a major in the army of Reze and serves as an advisor to the Silver Armored Brigade. He is a young, serious man who is almost always seen wearing glasses, a trait that has become his personal trademark and has earned him the nickname Glasses from his comrades, who also sometimes call him Ike. His appearance, which includes blue eyes and brown hair, stands in contrast to the more eccentric and varied styles of his subordinates, among whom he is typically the voice of reason. Despite his composed and rational demeanor, he is known to be self-conscious about a receding hairline, which has led to the occasional unflattering nickname, Baldy.

Four years before the main story begins, Isaac held the rank of lieutenant and was investigating a new drug trafficking case in Reze and its neighboring countries. His failure to capture the ringleader of the operation resulted in his demotion. Later, as internal political concerns grew regarding Staria Reze, Isaac was promoted by two ranks and dispatched to serve as the third advisor to the Silver Armored Brigade, following the deaths of his two predecessors in the line of duty. His glasses are non-prescription; he began wearing them after interpreting a comment from Staria Reze about his "stuck-on glasses face" as an order, and he found that wearing them softened his naturally harsh gaze.

Isaac is defined by his strong sense of duty, rationality, and pragmatic thinking, which makes him a highly trusted and respected commander. Within the brigade, he is the sensible man who often bears the brunt of his comrades' antics, acting as a mediator and a stabilizing influence among a group of misfits. While his strict and serious nature might seem cold at first, he is deeply loyal to his allies and demonstrates considerable courage and resilience in battle. He plays a crucial role in managing the logistics and strategy for the Silver Armored Brigade.

During a major conflict at a military training ground, Isaac engaged in battle with the Platinum Tiara. With strategic support from Crimson, he was able to emerge victorious. However, following this victory, he was briefly placed under a hypnotic suggestion by Crimson. Later, during the assault on the royal capital, Isaac took command of the brigade. He survived a fierce battle against Orto Zola and was the one to deliver the final, fatal blow to the mortally wounded dragon Borgius. After the war, he sustained injuries in a battle against the Winged Bloodline but survived and received treatment, subsequently living under Crimson's protection alongside Staria Reze.

Isaac is not only a capable strategist and leader but also an excellent swordsman. His combat prowess was demonstrated when he was able to deflect multiple magic bullets with his sword during his battle against the Platinum Tiara. He commands high levels of trust and loyalty from his subordinates and maintains a key relationship with Staria Reze, as well as a complex one with Crimson, who is both an ally and a source of manipulation. He develops a strong, dependable bond with his comrades over the course of the story, becoming a pillar of the group through his competence and reliability.