Masashi Yūki is a first-year student at Seido High School and a member of the baseball team, wearing number 15 on the first-string roster during the summer tournament of Ace of Diamond Act II Second Season. He attended Akadou Junior High and later Kokubun Senior, where he was a highly regarded fourth batter, known for his power hitting. His older brother is Tetsuya Yūki, the former captain and clean-up hitter of Seido, and Masashi follows in his footsteps by enrolling at Seido with the goal of reaching the major leagues. He takes baseball very seriously and possesses a fiercely competitive nature, often radiating an intense, fiery aura when watching others play, especially against fellow first-years. Upon his arrival at Seido, he expressed that he did not mind where he was placed in the team's lineup, showing a selfless and determined attitude focused on contributing to the team's success. His primary role is as a promising slugger, and his development involves adjusting to the high level of high school baseball while carrying the legacy of his brother. Notable abilities include his raw batting power and his intense focus during games, which make him a key asset for Seido's future. Throughout the story, his relationships with his older brother Tetsuya and his teammates help shape his growth as a player and as a person.