Shinagawa is the protagonist and "yankee-kun" of the Yankee-kun to Megane-chan (Series). Throughout the manga he is always being caught up in Adachi's and the other students' problems. Despite the series being instigated by Adachi and her desire to change, Shinagawa also changes drastically. He is Adachi's Student Council Vice President.
Daichi is a young man of average height with spiky light-colored hair and high eyebrows. He wears his school's uniform which consists of a blue long-sleeved shirt, a dark-colored blazer, a belt, pants and black shoes. He wears earrings and a necklace for accessories and a necktie is tied loosely around his neck. He also has a habit of opening the top buttons of his shirt.
Among the members of his group, Shinagawa is the only one who can be considered "normal" as he finds the idea of his friends odd and insane. At first he was always at the men's restroom hiding from everyone and make it his spot refusing to talk to anyone and have friends. Daichi is also a short-tempered delinquent who does not like to be bothered and has no plans for his high school life. That is until he meets Adachi, a former delinquent who convinces him to change his path else he might regret it like she did. At first he is merely pestered by Adachi into becoming more active in school, but gradually changes throughout the series: from small things such as not smoking and avoiding class to even becoming co-vice president of his classher is a lawyer while his father, whose personality is similar to his, is a renowned doctor. Later in the series, it is revealed that due to innate talent and his being very hard working, he was one of the top students in a reputable private elementary school. Upon realizing that delinquents were bothering his number one rival in studies, Shizuku Hachioji, interfering with her ability to study, Shinagawa became directly involved to save her. His involvement in a fight with those delinquents resulted in him being kicked out of the school.