Iori Matsuo is a nineteen-year-old college student who appears as a supporting character in the film Blood-C: The Last Dark. He is a member of Srat, an underground organization that opposes the activities of a mysterious group operating a private academy. Within the group, Iori takes on the specific role of driver, using his own car for their operations. While his fellow members Shun Fujimura and Hiro Tsukiyama possess notable hacking skills, Iori has very little aptitude for computers or the internet, relying instead on text messages to gather information from acquaintances. His primary partner within Srat is his junior, Shun Fujimura, and the two often work together as a duo.

Iori is driven by a strong, if self-styled, sense of justice. He describes himself as an extremist of the net and a journalist of the net, framing his actions as a personal crusade. His personality can be brash and troublesome at times, and he has a tendency to be cheesy, but fundamentally he is a decent and caring individual. Despite frequently arguing with other members, particularly Yanagi Haruno, he shares a genuine bond with them. He deeply values his companions, and this loyalty is demonstrated when he does not hesitate to use his beloved car to crash through the auditorium doors at the private Tsuji Academy, putting the vehicle at serious risk to rescue Saya and a young girl named Mana from a life-threatening situation.

His relationship with the protagonist, Saya, is initially tense. He rejects what he perceives as her cold and dismissive attitude towards the group. However, his perspective changes when he witnesses Saya's willingness to risk herself to protect him and the others. He honestly admits his initial misjudgment, and they are able to become close friends. He also maintains a quarrelsome yet fond dynamic with his fellow Srat member, Yanagi Haruno. This relationship deepens significantly by the story's end, as it is stated that they are to be married a month after the main events. In the epilogue, Iori is shown living with Haruno and running a small café together. As a driver, his most notable ability is his skill at operating a vehicle, a talent that proves critical to the team's survival. His appearance is characterized by black hair, black eyes, and accessories like earrings and a dog tag.