Orphaned young, Itsuki Suwabe works part-time alongside his younger sister Nagisa at Card Capital, raised by the shop's owner, Misaki Tokura, whom he deeply respects and gratefully views as a parental figure. This history contributes to his reserved and initially timid nature, often feeling overshadowed. A dedicated Vanguard fighter, Itsuki pilots the musical idol-focused "Bermuda Triangle" clan. His early playstyle mirrors his personality: passive, overly cautious, and undermined by profound self-doubt, leading to significant anxiety before and during matches, sometimes causing physical discomfort.

Itsuki's journey centers on overcoming this self-doubt through Vanguard, driven significantly by his rivalry with the highly skilled but arrogant Kyou Yahagi. Kyou's initial dismissal forces Itsuki to repeatedly confront his fears. A devastating loss to Kyou in a crucial match becomes a pivotal moment. Bolstered by friends like Aichi Sendou, and reflecting on his desire to protect Nagisa and repay Misaki's kindness, he finds renewed determination. He refines his Bermuda Triangle strategy, shifting from pure defense towards a more balanced approach that effectively utilizes the clan's strengths.

Subsequent encounters with Kyou showcase Itsuki's burgeoning confidence. He begins initiating plays proactively and standing firm under pressure. While not always victorious, these matches demonstrate marked improvement in both skill and mental fortitude. His dynamic with Kyou evolves from one-sided intimidation to a complex relationship where Kyou acknowledges Itsuki's growth. Itsuki maintains strong bonds within the Card Capital group, particularly with supportive friend Aichi. His protectiveness towards Nagisa remains a core motivation.

By the conclusion of his major arcs, Itsuki has transformed into a more assured fighter. Though nerves persist, he now faces challenges head-on, actively pursuing greater strength to support those important to him. He continues working at Card Capital and competing in tournaments, embodying persistent effort and gradual self-improvement.