Little Sister in Training
"You owe me donuts."
Renshuu "Renshiro" Shirokawa is the little sister of Mashiro. She is a high school student working part time at her family's company, Shirokawa Corporation.
When the company was under attack by Knell's gang, she quickly grabs the unfinished device she was working on, The Quarter, which allows the user to reflect projectiles. Unlike her sister, who is very skilled with using swords, she only received a few small lessons from her father to defend herself, using a wakizashi blade. She eventually gets surrounded by criminals, but she was rescued by her older sister. She then gives the Quarter to Mashiro, as she knows her older sister can use the device to rescue their kidnapped mother.
After the events of The Quarter Game, Knell's gang was spared and they started their own restaurant. Renshuu was inspired by their efforts after their failure, as she wants to start her own Donut Shop, so she visits them frequently.