Tsurugi is 31-years-old, the eldest of the Yagami sisters, and is a teacher at Sasami's high school. Despite her age, she has the appearance of a little girl and has the personality of a middle-aged man. She often plays adult games in the staff room. Her name is based on Kusanagi no Tsurugi. Later it is revealed that she is Amaterasu herself, who became fed up of her power and passed it on to Sasami's distant ancestor.
Source from Wikipeida