Taishi Miwa, a recurring character across multiple anime seasons and related media, attends Hitsue High School. He presents a carefree, comedic attitude that often irritates others yet remains fundamentally endearing. Beneath this exterior lies deep loyalty and concern for his friends, especially Toshiki Kai, his friend since elementary school. Initially portrayed as Kai's sole childhood friend, this changes with the introduction of Kouji Ibuki in the Neon Messiah movie.

Miwa acts as Kai's confidant and sparring partner, using cardfights to sustain their bond. He also manages Card Capital during others' absences. A skilled cardfighter, he wields a yellow Vanguard circle and employs a Kagero deck focused on retiring opponents' grade 0 and 1 units, featuring Garnet Dragon, "Flash," and Salamander units. He uniquely retains older cards like Undeux, differing from standard Kagero strategies. The manga depicts him using a Nova Grappler deck that rests rearguards for power boosts, utilizing Miss Splendor to prevent interceptions.

Miwa experiences significant emotional arcs. In season 3, he deduces Kai's role in spreading Reverse fighters and confronts him, resulting in defeat and Miwa's Reversal. While Reversed, he serves as Kai's enforcer, defeating underground fighters to isolate Kai's conflicts. After losing to Misaki Tokura, he reverts to normal but wrestles with guilt over Kai's solitude. Season 4 sees him lose memories of Aichi Sendou following Aichi's disappearance. A cardfight with Kai—specifically Kai's Legion activation using Thing Saver Dragon and Blaster Blade Seeker—triggers Miwa's recollection of Aichi. During this time, he reveals plans to attend an elite university, aided by Misaki's tutoring.

Later, in G Season 2, he reappears as an employee at Card Capital, known as the "Sub-Sub-Manager," and participates as a guardian representing the Dragon Empire during the "G Quest."