When Tomoe joined Border, he was in elementary school, and the youngest recruit at the time. Having been inspired by Kakizaki's desire to protect citizens two years earlier on TV, he decided to join Kakizaki Unit.
With the rest of his unit, he fights Lamvanein alongside Suzunari First, Azuma Unit, and Arafune Unit. The battle ends with their victory, and Kakizaki Unit is the only unit not to have lost a member. He is given an Outstanding Service Award for his efforts.
His unit goes up against Tamakoma Second and Katori Unit in Round 5. When they enter the zone, he regroups with his team before approaching the two enemies. When Kakizaki Unit does confront the enemies, Kakizaki and Fumika begin shooting to pressure Katori Unit, while Kotarō flanks them. However, Kakizaki Unit is forced to retreat when they are lured into a position between the opposing teams. After Chika Amatori begins to destroy the buildings, the teams decide to head for Chika. Kakizaki Unit is stopped by Yūma. Kotarō attacks Yūma directly and is launched by Grasshopper up into the air, where Chika hits his arm with Lead Bullet. Pinned to the ground, Kotarō becomes the first to Bail Out after Yūma beheads him.