Kaoru Yamazaki is a central figure in the story, known for being an old friend and former high school junior of the protagonist, Tatsuhiro Satou. Their first encounter occurred during their junior high school years when Satou attempted to rescue Yamazaki from bullies. Although Satou was unsuccessful and got beaten up himself, the act left a lasting impression on Yamazaki, who subsequently respected him greatly and decided to join the literature club with him in high school. Years later, the two reunite when Satou discovers that Yamazaki lives in the apartment next to his in Tokyo.

Yamazaki comes from a family of farmers in Hokkaido, where his life was rigidly planned out by his relatives, dictating the ages for marriage and having children. Rebellious against this predetermined path, he moved to Tokyo to pursue his dream of becoming a game creator, enrolling in a vocational school for game programming and design. He is a heavy otaku, obsessed with bishoujo games, hentai, dojin, and galge, and is often cited as the person responsible for introducing Satou to otaku culture.

In terms of personality, Yamazaki is a complex and volatile individual. He has a flamboyant and argumentative nature that often gets him into trouble, and he possesses a hair-trigger temper, especially when drunk, leading to arrogant rants. He openly declares himself a misogynist, claiming to prefer fictional two-dimensional girls to real women. This distorted view of romance stems from a traumatic event in his childhood when he invited a girl he liked to a fireworks festival; she lied about visiting her sick grandfather but was later seen there with her actual boyfriend. Despite his loud cynicism, he is a hard worker, shown by his dedication to creating a gal game, for which he takes on all the programming and artistic duties while trying to push the lazy Satou to write the script.

Yamazaki serves as a foil to Satou, acting as both a supporter and an enabler. While he is significantly more functional than the hikikomori Satou, attending school and attempting to socialize, their friendship is often dysfunctional and marked by bickering. He is also very manipulative at times, as seen when he creates a female avatar in an MMORPG to trick Satou into forming an online attachment, doing so in an attempt to teach him a lesson about reality.

One of his key motivations is the completion of their eroge (erotic game) titled "True Love," which represents his last-ditch effort to succeed in Tokyo and avoid his fate back home. His key relationships include his senpai, Satou, whom he both respects and scolds, and a classmate named Nanako, for whom he develops real feelings, which temporarily makes him forget his preference for 2D women. He is also highly suspicious of Misaki Nakahara, correctly guessing that her motives for helping Satou are not entirely altruistic.

Yamazaki's development throughout the story is a bittersweet journey toward accepting reality. Despite his ambition, his game project ultimately fails to launch due to Satou's unreliability and his own limitations. His patience with Satou wears thin as his personal deadlines approach, and he is eventually forced to return to his family's farm in Hokkaido to take over the family business because of his father's poor health. In a poignant conclusion to his arc, he abandons his dream of becoming a game developer, accepting the responsibilities of adulthood. His notable abilities include drawing in a manga art style, game programming, and illustration.