Kyousuke Irie is the head doctor of the Irie Clinic in the village of Hinamizawa. He also serves as the manager and coach of the local children's baseball team, the Hinamizawa Fighters. Born on December 3, he is in his late twenties to early thirties. At a young age, his father's deteriorating mental state and violent episodes inspired him to become a doctor and study the brain. He grew up in poverty and was known in his neighborhood as Doctor Kyousuke-kun, often playing the role of a doctor with his childhood friends. He studied under a neurosurgery expert and was expected to become a very successful physician, but his career took a dark turn when he began practicing psychosurgery, a controversial form of brain surgery that had fallen out of public favor. His support for and performance of lobotomies without patient consent led to allegations of inhumanity, and he was effectively banished from the medical academic society in Tokyo.

Seeking a chance at redemption, Kyousuke was recruited by Miyo Takano to research Hinamizawa Syndrome. He was drawn to the project because it aligned with his belief that "evil" behavior could be caused by brain diseases. He moved to Hinamizawa to run the clinic and conduct research under Takano, all the while unaware of her true, destructive intentions. Through his work, he managed to dissect a live specimen and develop an experimental cure for the syndrome. He tested this cure on the terminally ill Satoko Houjou as a way to avoid being forced to dissect her; the treatment succeeded, though Satoko required daily injections and regular checkups.

On the surface, Kyousuke presents a cheerful, caring demeanor. He makes house calls, tends to the villagers' health, and is highly respected in the community despite his youth. However, he harbors a severe maid fetish, which manifests in his half-joking comments about marrying Satoko when she grows up and his eagerness to chase after Rika Furude whenever she wears an Angel Mort waitress uniform. This perversion is a notable quirk that contrasts with his professional image.

Beneath the amiable facade lies a calculating and morally conflicted individual. Kyousuke is capable of making difficult and ethically questionable decisions to further his research or protect his work. He is described as mellow and easy to manipulate by others, particularly Takano, but he also shows genuine compassion for the children of Hinamizawa. He attempted to alleviate Satoshi Houjou's home stress by recruiting him for the baseball team, and he later expressed interest in adopting Satoko. His relationship with Rika is one of protectiveness; aware of the cycles of death she endures, he bears the burden of that knowledge while struggling against the larger forces controlling the village.

Kyousuke's role in the story is that of a supporting figure whose medical expertise and secret research tie directly into the central mysteries of Hinamizawa Syndrome. He works behind the scenes to manage the syndrome, develop countermeasures, and occasionally assist the main characters, all while operating under Takano's orders. His development is marked by a gradual shift from a disgraced, redemption-seeking scientist to someone who actively tries to protect the villagers, culminating in the creation of a functional cure. His notable abilities include advanced knowledge of neurosurgery and psychosurgery, a deep understanding of Hinamizawa Syndrome, and the practical skill to synthesize an experimental treatment.