In the Gokusen guidebook, there was a short story about Shin and Kumiko meeting while they were kids.

Shin is riding his bike when he sees two men on a motorbike stealing a purse from a helpless woman. He tries to catch up with them, but he just can’t pedal fast enough. Then all of a sudden, a girl (Kumiko!) easily runs up next to him and hops on the front of his bike, taking over the pedaling and giving him a nice panty shot in the process.

They still can’t keep up with the robbers, so Shin gets off the bike and Kumiko rides on ahead without him. When he finally catches up, he finds a note on his bike with the robbers lying on the ground all beat up, just like how adult Kumiko leaves her not-so-innocent victims.

Unfortunately for the police, Shin can’t give them any details about who the girl was who beat up the thieves. All he can remember is her cursing and panties! Neither Shin or Yankumi remember anything at all about this incident.