He had short and fairly messy hair which, according to the novel description, was darkish-brown in colour. Masato was always shown wearing the green Kokuyo uniform since he spent most of his time at school.
Masato was a person who hated violence and always tried to solve problems with peace. This trait motivated him to change the environment of Kokuyo Junior High, that had became known as a bad school where negative rumors won't stop amongst the neighbors, and make it a better place.
He's very dilligent and devoted to his duty as student council president and worked hard to change the school, making his friends in the student council follow him. But as Masato's hard work didn't blossom, they left him until he was the only one remaining. Masato drastically changed after Mukuro Rokudo along with Ken Joshima and Chikusa Kakimoto transferred to Kokuyo. He became distraught after most of the students in Kokuyo decided to follow Mukuro out of fear and felt that his hard work had gone to waste. He started to use violence, like when he beat up one of the students who told him to stop and continually did so to the other students who went against him, which made for a heavy consequence for himself once Mukuro, who was fooling him with an illusion, decided to lift the illusion.
He, along with the student council vice president, Kabane were discussing the conditions around Kokuyo school while patrolling the school and erasing graffiti when Kabane questioned their hard work. Masato defended the work they were doing, saying that it wasn't pointless. In the middle of this conversation, the two were interrupted by Yaginuma, one of the thugs in school, who told him to give up and started to beat him up.
After he was done, he said that he was bored and he is going to start a fight with the three transfer students, Mukuro Rokudo, Ken Joshima, and Chikusa Kakimoto. Masato tried to stop him but Yaginuma ignores him, due to a rumor that he had heard, saying the three of them were strong. As Yaginuma and his group left, Masato decided to follow them to try and stop the fight.
In Kokuyo Land, Masato charged into the building and went into the theater, only to find Yaginuma and his group already beaten, in a scene that he described as hell. Mukuro questioned what he'll do, and Masato pleaded him to stop, to which Mukuro refused, since Yaginuma was the one who initiated the fight. He suggested Masato to fight to protect his friends. Masato hesitated until Ken brought a tied-up Kabane, who also entered the building out of worry. When Ken was about to beat him up, Masato finally rushed to Kabane and for the first time, used violence by punching Ken and running away from the building with Kabane. Outside the building, Masato asked Kabane to forget about what he just did. A day after the incident, Masato was haunted by the fact that he used violence, but Kabane, however, cheered him up and lightened his mood again. This wouldn't last, as a few moments later Yaginuma approached them and told Masato that he will follow Mukuro as he was afraid of him. Yaginuma advised him to not stand up against Mukuro or he would be as good as dead. Masato became more distressed when Yaginuma pointed out that all of his hard work had no meaning behind it from the start. Masato finally reached his limit once Yaginuma told him that he was even lower than a worm, which enraged him to the point, where he started punching Yaginuma over and over while Lancia and Mukuro watched from the rooftop.
A few weeks passed, and the condition of Kokuyo Junior High had became much better, with all the students listening to Masato, making the school environments clean and comfortable. Masato entered the student council room, to find Kabane with an uneasy expression. Kabane asked him weather he used violence to make this change, citing a rumor that he had heard from his friend, Kashima. Masato locked the door and asked Kabane about what he had done wrong. He admitted that he indeed used force to make the delinquents obey to him, and punched Kabane, threatening him to keep quiet about it.
The next morning, Masato and several students found photos of him punching Yaginuma on the message board, Masato hastily ripped them off before his reputation could get tarnished. Fortunately for him, the photo only showed his back and not his face. A few moments later, Yaginuma delivered news that Mukuro and his friends had came back. Looking back at the photos, Masato planned to make Mukuro a black sheep, a sacrifice, in order to make Mukuro obey him by force so that he won't ruin his ideal school. He, Yaginuma and a few others went to Kokuyo Land to confront Mukuro. However, things started to go wrong, as Mukuro quite easily, took down everyone who attacked him. At this point, Kabane revealed himself, much to Masato's confusion. Masato then noticed the kanji "six" on his right eye. Masato finally realized that the Mukuro who was before him was an illusion.
Mukuro explained that he had been by his side all this time to mislead him. Masato checked his cell phone, and to his horror, realized that only three days had passed since his confrontation with Mukuro and Yaginuma. Mukuro continued to explain, saying that that Masato's ideal school and the delinquents being obedient to him, were all illusions that Mukuro had placed in his mind.
Mukuro showed his right hand, and told Masato, that if he snapped his fingers, the illusions of the delinquents being obedient to Masato will disappear and the only thing that will remain, will be the memories of them being abused by him.
Masato shocked by this, realized that Yaginuma and his group never truly followed him because of fear of his strength, but because of Mukuro's illusions. Terrified about what will happen when Mukuro released his illusion, Masato begged Mukuro to stop. But Mukuro playfully ignored him and snapped his fingers, much to Masato's horror.
Later, Chikusa brings in Futa, and Masato could be seen, unconscious and barely breathing, with broken bones and bruises all over his body, and his face no longer recognizable. Mukuro comes close to him and whispers that humans are wonderful. After that, what happened to him next remains unknown.
- Masato means "man in truth" in Japanese while his Family name, "Hitsuji", means "sheep" in Japanese since the kanji 日辻 which is only a name is similar with 羊, which means sheep. Mukuro chose him as a 'sacrifice' because he found his name interesting.