Ikuko "Koiso Ryoko's Friend #A" Hirayama

About

Koiso's best friend, and the only "normal" friend of Kitano. She's very sensitive, and she's the only one to openly notice how much being friends with Kitano has changed Koiso and Shirataki.

Her name has come to one of the series' running gags, with Kuroda repeatedly calling her Koiso Ryoko's Friend #A. Her first name is first used about one-fourth into the story, and her family is not revealed until the very end of the series. The author had even thrown a contest to let the readers guess the family name (only one reader has correctly guessed what her surname is).

Wikipedia