He is voiced by Daisuke Sakaguchi in the Japanese version of the anime and Greg Ayres in the English version.
Yamazaki is a young man in his early twenties of average height and build. He has black hair and brown eyes that he wears a pair of glasses over. His outfit tends to vary, though it usually consists of a collared shirt and trousers.
Yamazaki is an otaku obsessed with bishoujo, hentai, dojin, and galge. He's spent most of his life on his family's farm but rebelliously moved to Tokyo when he learned of what his family had planned for him, having been born to a farming family where everyone lived their lives as planned out. His flamboyant personality and tendency to become really argumentative pissed off most students at the school leading to him to get frequently beaten up, which in turn led to Satou standing up for him. This left an impression on Yamazaki, and the two became friends afterwards.
Yamazaki is openly misogynistic, outright stating that he prefers fictional girls to real ones (though this is void whenever the girl he likes calls him). This and his displeasure with fireworks stem from his childhood; he asked a girl he had a crush on if she wanted to see the fireworks festival, but she said she couldn't because she had to visit her grandfather in the hospital. Yamazaki ultimately decided to go by himself, but discovered that she had lied and came to the festival with her actual boyfriend.
Yamazaki is also very ambitious, shown when his dreams for his and Satou's game development company are shown. He also works hard, in sharp contrast to Satou, as shown when he takes up all the game development duties (save for writing) for his and Satou's Gal Game: True Love. When he gets drunk, he also gets very arrogant, and his temper surfaces, particularly in regards to his aforementioned misogynism, describing women as "filthy whores".
Yamazaki can also be very manipulative; when Satou starts playing the MMORPG Ultimate Fantasy, Yamazaki creates a female avatar named Mia, and after Satou ended up developing a crush on the person, he reveals the truth to him. However, this deception was ultimately well-meaning i.e. it was to show Satou that he can't live the life of a hikkikomori forever, and whether he likes it or not, there will come a time when he'll have to do something about it. After Satou's freak-out, however, he acknowledges that he may have gone too far, though he does ultimately convince Satou to quit.
As the series progresses, Yamazaki's patience with Satou wears thinner and thinner; initially, he puts up with Satou's jerkish behaviour through impossible circumstances, though he isn't above calling him out for being an idiot. In the weeks leading up to him moving away, however, his short temper starts to show more and more, to the point that he explodes at Satou via the slightest provocation and second-guesses practically everything he does, due to a mixture of Satou's repeated failings and his own personal problems.