Mayumi Tanaka

She belongs to the same class as Chiyo Kurihara and Kiyoshi Fujino, and is a member of the school's Go Club.

The friendly character of Mayumi Tanaka and her good friendship with Chiyo is shown when she expresses concern about Chiyo when gloomy. While Chiyo relies on her for advice and mental support as a good confidante, Mayumi is a non-confrontational person and tries to avoid fights - thus inadvertently depending on Chiyo for physical defense and/or protection. Though she does not seems antagonistic towards the boys, Mayumi is apathetic towards Chiyo's interest in Kiyoshi & the enrolment of the five first-year boys in the school. In contrast to Chiyo's love of sumo wrestling, Mayumi shows an interest in clothes and magazines. However, Chiyo is completely oblivious to her disinterest in sumo wrestling.

(Source: Prison School Wiki)