Kusozakoboukou Oshii-san

The high school freshman Mamoru Kishino was expecting a fateful encounter and he met the pretty girl classmate Koko Oshii, and happened to know her secret. To his surprise, he found out Oshii-san's bladder is super weak...!! To not leak this secret, Kishino swore to protect Oshii-san('s bladder).

(Source: Nihon Bungeisha, translated)