Matsui is a member of the Shinpaku Alliance as well as its flag keeper and in Class 2-A
Personality
Matsui shows great loyalty to the alliance's leader, Haruo Niijima. He appears to be the flag keeper of Shinpaku as he is almost never seen without it, showing great dispair during the DofD tournament when it was supposedly lost after Natsu Tanimoto's boat crashed into the rocky cliff surrounding the island it took place on. His loyalty is great enough to stand by his captains even during the final battle against Ragnarök.
Abilities
He has only been shown fighting twice; the first during Kenichi Shirahama's fight with Ryūto Asamiya and the second
during the fight with Fortuna. Matsui made a slashing move with his flag at Ryūto calling the attack Shinpaku Slash. Because of this, it is assumed that Matsui may be experienced in fighting with weapons. He also attepted to use this attack against Fortuna, but the master blew him away, saying he was not interested in him.