Mutsu is a small child with short black hair that covers his forehead. He wears a long sleeved shirt with 13 on it and a line above it and two lines on each sleeve. He also wears pants that are rounded up to the knees.
Mutsu is a calm child that followers his families teachings.
Mutsu was able to defeat his upper classmen and rise to the top of his school as a third graders. He had once told Yamato that he face a tough guy who was manipulated by a smart guy.
Mutsu is introduced by Musashi along with the rest of his family. He then begin their morning assembly by punching a punching bag and saying what they have accompliced. He then turns around and looks at Yamato, when he was drawing his manga. He then looks at Yamato as he wonder why he doesn't want to be strong. Yamato tells him and his brothers to read his manga and he does. He and with his brothers think that it sucks. When Yamato says that he will have his friends read it, Mutsu along with the others tell him that his friends would die if they read it. When Musashi wonder how Yamato turns out like this and remember what he did for him, Mutsu look shocked after hearing it.
Later that day, Mutsu is eat dinner with his brothers while watching one of Musashi's fights. After eating, He and his brother (except Yamato) then preform a training exercise. Later that night, is asleep along with Shinao and Hyuuga. After Musashi and Yamato's talk, Mutsu, Shinao and Hyuuga are noticed by Musashi that they are awake and that they had the same idea as Musashi.
Then next day after Yamato returns from school, He alond with Musashi, Shinao, and Hyuuga head out to eat Yakiniku. After eating, he along with his brothers head to karaoke. When Musashi comments on how Yamato should have come with them, Mutsu doesn't says anything.
The next day, Mutsu is watching Musashi juggle a punching bag by punching it. When Yamato starts to act weird, Mutsu asks why he is acting weird. Mutsu then hears Mizuho saying that she wants to fight. When Mizuho tries to fight Yamato, Mutsu laughs as Mizuho punches do nothing to Yamato. When Mizuho is warned about the guy who is killing girls and the she will kick his ass, Mutsu along with his brothers says that its impossible.
Mutsu is incredibly strong as he was able to defeat an upperclassmen at school. He can even defeat a guy more then twice his size.