Sakaki Shinobu

Kida's classmate and Touko's best friend. A sharp-tongued and quick-witted girl who holds the position of class representative. Having known Touko all of her life, Shinobu has taken it upon herself to look after her best friend and punish anyone that crosses her. Unfortunately, this has done more harm than good for Touko, even if Shinobu is for the most part unaware of it. Her devotion is so strong that her classmates suspect that Shinobu and Touko might be romantically involved.

Edited from Wikipedia