Ikuko Hirayama  

, 平山郁子

Koisos closest companion and Kitanos only "normal" friend. She's highly sensitive, and she's the only person who openly acknowledges how much being friends with Kitano has affected Koiso and Shirataki. Her name frequently appears in one of the series running jokes, with Kuroda often referring to her as Koisos Friend #A. The first mention of her given name occurs about one-fourth into the story, while her family remains unknown until the very end. In fact, the author even held a contest for readers to guess her surname, but only one person correctly guessed it.