Zenjiro is the teacher in the TV program Child's Toy, which also stars Sana. Although Child's Toy is mentioned in the books, the teacher is a very different secondary character called Mr. Yonma. Zenjiro is hyper and loud, with large buck teeth; he is apparently based on a real-life TV personality. Both Mr. Yonma and Zenjiro are from Osaka.

Sana Kurata has a charmed life. Not only is her mother is a famous, award-winning writer, but she's the star of the hit TV comedy "Child's Toy" while still in the fifth grade. But Sana's biggest concern is Akito Hayama, a pint-sized hellion who's organized the boys in their grade-school class into a mass of unending noise and violence. With the help of Rei, her manager, chauffeur and "pimp," Sana is determined to win back control of the classroom from her new arch-enemy. But as her crusade continues, Sana moves further into Akito's life, and finds that he might not be entirely bad...or entirely safe.
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(Source: Anime News Network)

Zenjirou