Matsui is the flag keeper of the Shinpaku Alliance, he is very loyal to the alliance's commander Niijima.
Matsui wears a dark school uniform with a cap, and has bushy hair which protrudes sideways. While the basic elements of this design haven't changed, how he is drawn has, as Matsui was initially depicted as a tall and quite imposing figure, but over time his design gradually softened and he started to appear shorter. He is almost always seen carrying the banner of the Shinpaku Alliance.
Matsui is a loyal and enthusiastic member of the Shinpaku Alliance, a union of teenage martial artists and other individuals who serve Niijima Haruo's agenda. While he is quite a minor figure, he has managed to come across as a particularly emotional person who usually wears his heart on his sleeve and is totally devoted to Niijima and the Alliance.
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