No. 21 (21号 ) is a minor antagonist featured in Stay Cool, Kobayashi-san!: A River City Ransom Story. She is a leader of the evil organization and is looking to change a tragic event in the future.
Background[]
No. 21 will first appear upon defeating the thirty of the green fighters and will briefly introduce herself before asking Kobayashi to fight. Upon being defeated, she mentions that her loss was inevitable and says goodbye to Kobayashi knowing that she can finally return back to her time period.