Prince Kaiser stands as the central rival within the Duel Masters Win narrative. A traumatic childhood event defines his past: his mother perished in a space station crash caused by Abyssbell = Jashin Emperor. Bolshack Kaiser, Dragon Emperor of Booming Flame, rescued him during the disaster and became his constant bodyguard. This guardian appears in human form wearing a mask that acts as a power limiter, connected to Kaiser's ring—a keepsake from his mother.

He wields significant authority as the young CEO of the global Ryujin Corporation, which he expanded twentyfold after inheriting it from his father, and as president of Maihama Academy. His childhood involved surviving numerous assassination attempts from corporate rivals, earning him the name "Immortal Kaiser." These experiences fostered deep distrust of adults and a belief that they exploit the young, driving his ambition to create a world governed by strength and talent.

Kaiser exhibits a cold, ruthless personality. He views defeated opponents as worthless, firing employees who lose to him and burning their decks—an act he performed against Win Kirifuda's father, Guts Kirifuda, igniting Win's rivalry. He despises dueling for mere enjoyment, prioritizing victory above all. Despite this, he maintains certain boundaries; he privately shunned Fargo Kasumigaseki after witnessing his cheating but took no public action. His obsession with eradicating Jashin, stemming from his mother's death, led him to manipulate Karen into working against Win and Jashin-kun.

As leader of D4, Maihama Academy's strongest duelist group, he tests members' worth through high-stakes duels. He demoted Bowie Obochiyama after losses, replacing him with Karen following her victory using her new trump card, Chaosmantra, Holy. He later disbanded D4 entirely in a fit of rage. His character arc intensifies when DARK MATERIAL COMPLEX manipulates him by mimicking his mother's voice through Ugata, falsely blaming Jashin for a zombie outbreak at the academy. This deception drives Kaiser into madness, compelling Bolshack Kaiser to evolve into Bolshack Bakuterasu, Dragon Emperor God. Kaiser absorbs this power, creating destructive shockwaves and targeting Win to kill Jashin.

During his final confrontation with Win, Kaiser's delusional state manifests as dark mana infestation. Despite initially overwhelming Win, he loses when Win strategically uses Abyssbell = Jashin Emperor's abilities to survive Kaiser's Apocalypse Day attack. Bolshack Bakuterasu perishes in the aftermath, causing Kaiser to regain his sanity and mourn his loss. Though weakened and vulnerable, he avoids zombification and assists by ordering Fargo to stall Ugata while Win pursues other threats.

In duels, Kaiser employs a fire civilization-focused Bolshack deck renowned for speed and power, featuring key cards like Volcanic Dragon Emperor Bolshack Kaiser, Supreme Flame Dragon Bolshack Rider, and Dragon King God Bolshack Bacteras. His strategies leverage Shield Triggers, G-Strikes, and uncanny luck in critical draws.

His relationships are defined by rivalry and hierarchy: Win Kirifuda transitions from admiration to bitter enmity after Kaiser humiliates Guts; Bowie Obochiyama suffered harsh criticism and eventual expulsion from D4; Karen joined D4 under his command but faced suspicion regarding her loyalty. Post-defeat, his documented state is one of fragile recovery.