He is a friend of Toshiki Kai; they have been friends since they were young and he is Kai's only friend. (However, this plot seems to have been amended in Neon Messiah, with the appearance of another childhood friend, Ibuki Kouji.) He understands Kai and plays Cardfight with him. Unlike Kai, he's much nicer and friendly to people, and loves to joke. He also goes to the same school as Kai.
Like Kai, despite continuous reference to 'Miwa' both in-series and out even by friends, Miwa's given name is in fact Taishi.
Miwa is now a senior with Kai. During his fight against Shingo, Shingo underestimated Miwa's skill and Miwa manages to wipe him out easily.
He later faces off against Misaki during the regional qualifiers, using his deck full of Seal Dragons, but falls short of defeating her.
When Kai returned to Japan after travelling around the world following his Reverse, Miwa confronts him, having learnt of cardfighters suddenly becoming strong and gathering more into their group worldwide from the net. He was also able to deduce that Kai was the cause of the growing chain of the Reverse fighters, despite hoping that he was wrong. Saddened upon knowing of the expected truth from Kai, he challenged Kai, using his old Kagero deck with Dragonic Overlord the End, only to lose to Kai's new ace card, Star-vader, Chaos Breaker Dragon. He subsequently gets Reversed as well.
After getting Reversed, he plays the role of being Kai's right hand, going off to defeat the underground fighters and even Jun so that they would not interfere with Kai's desire to fight Aichi.
He is seen with Kai at Tatsunagi Industries welcoming Aichi, Misaki, Naoki, Kamui, and Shingo. He challenges Aichi to a cardfight, saying he wants to test him, but ends up fighting Misaki. He manages to get the upper hand with Dauntless Dominate Dragon "Яeverse" but ends up losing to Misaki's new ace card, Omniscience Regalia, Minerva. After being defeated, he reverts to normal but feels sorry for Kai for being alone again.
When Aichi disappeared, Miwa also lost his memories of him. At school, Miwa was seen with Kai in their own vanguard club, where Kai challenges him to a match. During the fight, Miwa struggles to remember the person that Kai keeps on mentioning, but to no avail. As the match reaches to its climax, Kai rides Seeker, Thing Saver Dragon and continues to attack, but Miwa successfully guards all three barrages. Realizing that he must use his own power to recall the person that he forgot, he break rides Dragonic Overlord and attacks with full power. However, Kai manages to guard all those assaults, leaving Miwa close to giving up remembering the blurry images looming around his head. Then during Kai's turn, he activates Legion, calling Blaster Blade Seeker to the vanguard circle alongside with Thing Saver Dragon, triggering Miwa's memories of Aichi and thus beating him. In some part of a match, Aichi asked Miwa what is his plan and Miwa answered he is going to an elite university with Misaki. Misaki helped Miwa to pass the entrance test.
After the match, Miwa learns of a butler that has some sort of a connection to Aichi.
G Season 2
Miwa makes a reappearance in Cardfight!! Vanguard G: GIRS Crisis, during the "G Quest". He is seen as an employee at Card Capital, being called the Sub-Sub-Manager by Misaki, and as a guardian in the G Quest in the Dragon Empire.