30 years old. A guard at the Suzumiya Prison.
In his younger days, he used to be quite a player, and lived on the more shady side of society. Then he met Suzumiya Kazuo, a policeman, who helped him change his life around. Masato also met, and fell in love with, Mitsuko. However, she was Kazuo's girlfriend, and she turned Masato down, though Masato remained friends with both Mitsuko and Kazuo and buried his feelings for Mitsuko.
When Kazuo founded the Suzumiya Prison, he scouted out Masato to work for him, as one of his most trusted friends (also, because of Masato's past, he couldn't really find a stable job elsewhere). Since then, Masato has been one of the most dedicated employees of the prison.
When Kazuo died, he asked Masato to take care of Mitsuko for his sake, and Masato has done so, as a loyal friend. While he still has feelings for Mitsuko, he kept them buried and in check, especially as Mitsuko was very loyal to her deceased husband and did not warm up to any other man.