Kotarou Mizunuma

Kotarou participates willingly in his younger brother's bullying and does nothing to stop the harassment of Kiku. His limited screen time establishes him as part of the intimidating group surrounding Ryujiro rather than giving him a substantial individual personality.

He follows the confrontation as Takeru steps forward to protect Kiku. Unlike Ryujiro, however, Kotarou does not become the principal challenger.

The series provides little characterization beyond his involvement with his brother and the bullying incident.

Kotarou is not shown participating in a complete duel and has no established Deck, signature monster or LINK VRAINS avatar.

His role is entirely within Takeru's real-world memories. No special combat, hacking or supernatural abilities are attributed to him.

Kotarou appears in a flashback concerning Takeru's life with his grandparents after the Lost Incident. He accompanies Ryujiro and the other youths involved in harassing Kiku.

Takeru's decision to confront the group becomes significant because he is still struggling with the fear left by his childhood captivity. Ryujiro subsequently turns the confrontation into a duel.

The incident culminates in Takeru overcoming the fear triggered by Despair from the Dark and defeating Ryujiro, while Kotarou remains a bystander to the duel.

Ryujiro Mizunuma is Kotarou's younger brother. Kotarou accompanies Ryujiro Mizunuma during the confrontation involving Kiku.

Kiku Kamishirakawa is bullied by the group to which Kotarou and Ryujiro Mizunuma belong.

Takeru Homura intervenes to protect Kiku Kamishirakawa from Kotarou, Ryujiro Mizunuma and their companions.