Masamune Tooya is a 24-year-old man with orange hair and mint green eyes who stands 1.86 meters tall. He is a descendant of the scientists who created Aion, the artificial intelligence central to the world of Norn9, and hails from the same island as Natsuhiko Azuma. Orphaned at a young age, he grew up in an experimental laboratory where he was one of the few survivors of testing, an experience that left him with supernatural abilities and shaped his cautious, self-reliant nature. Because of his island upbringing, he possesses knowledge about the nature of the espers' abilities, the true purpose of the journey aboard the Norn, the identity of the world, and the mechanics of resetting events, information that many other characters lack from the outset.

Masamune is the captain of the Black Sirius and one of the three selectable love interests for the protagonist Koharu. He functions as the most reliable esper on the ship, acting as a voice of reason and a natural leader. He can often be found mediating disputes and performing administrative tasks to maintain harmony among the diverse and sometimes dysfunctional group of passengers. Despite his intelligence and wide-ranging knowledge acquired through avid reading, he is notably clumsy with his hands and not particularly skillful at practical tasks. He habitually wears gloves, which are related to controlling his power. His ability allows him to see past events simply by touching a person or object with his right hand, a talent that makes him invaluable for uncovering the history and secrets surrounding the group's mission.

Masamune's personality is marked by a strong sense of responsibility, maturity, and protectiveness toward others. He takes on an older brother-like role for the younger members of the ship, looking after them and offering guidance. He is often perceived as direct and assertive, with a tendency toward confrontation when he feels control or security is threatened. Beneath this exterior, he struggles with the trauma of his past and initially tries to maintain emotional distance from others, guarding his vulnerability. Over the course of the story, he gradually opens up, particularly through his deepening bond with Koharu. Their relationship becomes a significant subplot as they navigate their feelings amid the chaos and danger of their missions. He also forms a meaningful connection with Mikoto and comes to rely on the trust of those around him.

Masamune's development centers on learning to reconcile his painful history with his desire to protect the people he cares about, allowing him to move from a guarded, solitary figure into someone who can accept support and express his emotions more freely.