Iori Sendō serves as the student council president at Shujikan Academy and is the older brother of Erika Sendō. He is a being who has lived for a very long time, having been turned into a vampire intentionally. His appearance is notable; he gives off the air of a male model with blonde hair that reaches his ears and blue eyes, making him overwhelmingly popular among students regardless of gender, with countless girls having confessed their feelings to him.

Personality-wise, Iori is free-spirited and whimsical, possessing a smirk that reflects a nature which tends to treat everything as a matter of play. He has a unique personal logic that he presents as if it were an absolute law of the universe. Despite his calm and refreshing demeanor, which his sister Erika notes is merely a sample appearance that does not match his true contents, he is fundamentally not on bad terms with those around him. He frequently clashes with the quick-tempered Erika, but their relationship is generally good. Iori is a pleasant individual known for giving energetic speeches during school events, such as declaring to save the world together.

The character's background reveals that he was not originally a member of the Sendō family by blood but was adopted. He was made into a vampire by Kaya, the same figure responsible for Erika's condition. Over time, his relationship with Kaya deteriorated, leading him to leave the Sendō family and take up residence at Shujikan Academy, where he has served as student council president on and off over the years. A glimpse into his distant past involves a very old tree on the school grounds. This tree, said to grant wishes, was planted by Iori many years ago for a girl he liked, long before the academy even existed.

In the story, Iori plays a pivotal role by introducing the protagonist, Kohei Hasekura, to the secrets of the school, including the existence of vampires. He forcefully recruits Kohei into the student council, not as a random act, but as part of a deliberate plan. Iori observed Erika's unique reaction to Kohei's blood and understood that she was specifically attracted to it. His scheme was to bring Kohei closer to Erika, aiming to make Kohei her servant, thereby engineering their relationship. He is also one of the individuals who confronts Kohei with the reality of the supernatural, as seen when Kohei discovers him in the chapel drinking blood from a young woman.

Key relationships define much of his role. Beyond his dynamic with his sister Erika, he shares a long-standing acquaintance with Seiichiro Togi, a member of the student council's finance team. He has a noted difficulty in dealing with Amaike, as she is the daughter of a woman he once loved in the past. His relationship with his maker, Kaya, is adversarial, having soured to the point of separation from the family.

Regarding development, Iori's character arc involves his long existence as a vampire. At one point, he is offered the chance to live as a normal human but chooses not to, stating he still has things to take care of. Unlike his sister Erika in certain conclusions, Iori decides to continue living as a vampire because he still has work to do and wishes to enjoy his life further. Regarding his notable abilities, as a vampire, he possesses the supernatural traits common to his kind, such as extended lifespan and red pupils when revealing his nature, alongside a more manipulative and calculating intellect used to orchestrate events around him.