Ryuji Yoshioka attends high school as a classmate of the main characters and shares a long-standing friendship with Shota Tanoue dating back to elementary school. His family connection as the grandson of a kendo dojo master established his martial arts training, leading him to achieve third place at the National Middle School Tournament.

Physically, he possesses off-black hair that typically covers his forehead and matching dark eyes. He is most often seen in his high school uniform, maintaining a characteristically stoic expression.

Ryuji exhibits a cool, logical demeanor. Social interactions do not come easily to him, resulting in awkwardness within groups, and he actively avoids attention from school fangirls. However, his usual reserve subtly shifts in situations involving Miki Takekasa, around whom he displays behavior influenced by his developing crush.

His analytical approach, reflecting kendo discipline, extends to board games. During the school festival, his participation in a game of Dobble (Spot it!) with Miki and student council members reveals a difficulty with rapid identification and verbalizing responses under pressure, contrasting his typical calm.

His name, Ryuji (龍二), combines the characters for "dragon" (龍) and "two" (二), while his surname, Yoshioka (吉岡), signifies "lucky, good" (吉) and "ridge, hill" (岡).