Isaac Stern is a major in the Reze army and serves as an advisor to the Silver Armored Brigade. He is a young, serious man who is easily recognized by his glasses, a trait that has earned him the nickname Glasses from his comrades. Within the brigade, he is often the sensible, rational core of a group of much more eccentric personalities, frequently bearing the brunt of their antics while remaining highly professional. Despite his stern demeanor, he is deeply self-conscious about his receding hairline and is occasionally teased with the nickname Baldy. Interestingly, 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 to wear them, and he found they softened his naturally harsh gaze.

Four years before the main events, Isaac held the rank of lieutenant and was investigating a new drug trafficking ring in Reze and its neighboring countries. He was demoted for failing to capture the operation's ringleader. Later, as the political situation became more tense and Staria Reze was seen as a threat by those in power, Isaac was promoted by two ranks and dispatched to act as an advisor. He was the third person assigned to this role, as his two predecessors had both died in the line of duty. His primary function within the brigade is managing logistics, but he is also an excellent commander and a highly skilled swordsman, earning the deep trust of his subordinates.

In the field, Isaac has demonstrated significant tactical and combat ability. He led his unit during a battle against the Platinum Tiara at a military training ground, deflecting multiple magic bullets with his sword. With support from Crimson, his forces achieved victory, though he was subsequently placed under Crimson's hypnotic suggestion for a period. During the assault on the royal capital, he was entrusted with commanding the Silver Armored Brigade. He survived the intense battle against Orto Zola and was the one who delivered the final blow to the mortally wounded Borgius. After the conflict, he received treatment for severe injuries sustained during the fight against the Winged Bloodline and, alongside Staria, remained under Crimson's protection. His relationship with Staria Reze is one of loyal service, and despite her teasing, he holds her in high regard. Through his unwavering sense of duty, logistical expertise, and proven leadership, Isaac Stern stands as a reliable and respected pillar of the Silver Armored Brigade.