Masa is a high-ranking member of the Seto Group, serving as the right-hand man of its leader, Gozaburo Seto. He is physically tall, muscular, and handsome, with a distinctive afro hairstyle and a preference for wearing a suit and tie. He is almost always seen wearing sunglasses, so his eye color remains unknown. As a merman, Masa is one of the few merfolk who is not afraid of cats, having deliberately overcome that fear as a child. He also appears to be able to control the details of his human appearance, unlike many other mermaids.

In terms of personality, Masa is typically silent and reserved, rarely displaying strong emotions. He only becomes noticeably expressive when someone close to him is in danger. Despite his stoic exterior, he is consistently kind and helpful to the protagonist, Nagasumi Michishio. He is the only member of the Seto Group, aside from Sun Seto’s mother, Ren, who does not openly oppose Nagasumi’s engagement to Sun. He occasionally offers Nagasumi advice and protects him from Gozaburo’s attempts on his life. Masa is also fiercely loyal to his duties and to Gozaburo, but his sense of justice and calm demeanor set him apart from the more chaotic members of the group.

Masa’s primary motivation is his sense of duty as a member of the Seto Group. He follows orders reliably and works to maintain the safety of those under his protection, especially Nagasumi and Sun. He also shows a protective instinct toward his long-lost sister, Akeno Shiranui, even though he suffers from amnesia regarding their past relationship. His actions are driven by loyalty and responsibility rather than personal ambition.

Within the story, Masa serves multiple roles. He is initially introduced as Sun’s kendo instructor and later becomes Nagasumi’s math teacher, transferring to his school. His word problems humorously involve yakuza-style gang operations. Early in the series, Masa performs CPR on Nagasumi after the boy nearly drowns, a moment that is comically framed as Masa stealing Nagasumi’s first kiss. This incident becomes a recurring gag, with Nagasumi and his mother showing an inexplicable attraction to Masa, while Nagasumi’s father reacts with jealousy. Masa also becomes talkative when drunk, providing comic relief.

Masa has key relationships with several characters. He is a loyal subordinate to Gozaburo Seto, but unlike other members, he does not join in the boss’s hostility toward Nagasumi. He acts as a mentor figure to Nagasumi, offering guidance and saving him from harm. He supports Sun’s happiness in her engagement. Later in the series, it is revealed that Masa is the brother of Akeno Shiranui, a mermaid examiner. However, Masa has lost his memories of this connection, adding an element of mystery to his backstory. He cares deeply for Akeno and becomes protective of her when she is in trouble.

In terms of character development, Masa remains largely static throughout the series. His role is primarily comedic, centered on his quiet strength, the kissing gag, and his unexpected appeal to other characters. The revelation of his relationship with Akeno provides a rare layer of depth, but his core personality and attitude do not change significantly. He remains a steadfast ally and a calming presence amid the chaos.

Masa’s notable abilities include his skill as a kendo instructor, indicating proficiency in swordsmanship. He is physically strong and capable in combat, as befits a high-ranking yakuza enforcer. His ability to control his human appearance sets him apart from other merfolk, and his lack of fear of cats is an unusual trait among his kind. While he does not display overt supernatural powers, his presence alone often defuses or redirects potentially violent situations.