A wingback on Hiba Junior High and a defender on Tokyo Select; his jersey number is 6 on Hiba and 7 on the Tokyo Select team. Though he otherwise shows no respect for authority figures, he regards Shiina Tsubasa highly and is willing to follow his lead.
Despite being a delinquent, his profile notes that he dislikes vulgarity.
Masaki first appears in Vol. 10 BROTHERHOOD, along with the rest of Hiba's main defense, when Shou and others go to scope out their school. Rather than being on the soccer field, they are playing futsal.
Not unlike a defensive version of Sanada Kazuma in physical capability, Masaki is quick and efficient. Unlike Kazuma, however, he keeps a clear head on his shoulders and reads the flow of the game well; because of this, he can position himself effectively without prompting. He's keen on opportunities to steal, but isn't too aggressive. Importantly, he understands the teamwork aspect of soccer and does it well; he isn't shy of relying on others and can be depended on in turn.