Masamori was a very serious kid most of the time! He started training, mostly under his grandfather, as a kekkaishi when he was in grade school. He really worked hard to make his family proud and live up to expectations, and he took a lot of pride in succeeding at that. He did well in school, even while he was taking care of ayakashi at night. And he really tried to make sure that everyone in the family was happy with him. Masa has a huge sweet tooth, but his grandfather hates sweet things, so Masa never asked for them. (There's a flashback where Sumiko, Masa's mother, takes Masa's younger brother Yoshimori out to a cafe to have a parfait, and she's surprised when Yoshi tells her Masa likes things like that too.) And even though he has a lot of issues surrounding the fact that Yoshi is the legitimate successor, Masa tried to be a responsible older brother to Yoshimori and eventually Toshimori. He got along decently with other kids, but he never really had time to socialize. Everything was pretty centered around his family and making himself the best possible. He didn't start making contacts with the Shadow Organization until high school.