An is the 13-year-old sister of Kippei Tachibana, who also is a student at Fudomine Middle School (second year) and a regular member of their girls tennis club. Her more distinguishing physical traits are her pink barrettes (green when she's in sport clothes), and despite her boyishness, she's unafraid of using miniskirts, cute clothes or dresses, to pull off more feminine looks.

An cares very much for her brother and her friends from both the Seigaku and Fudomine teams, and tries to help them as much as she can. On the other hand, she has a rather explosive and even downright vicious temper that leads her to either get in trouble for the sake of others (as proved by Sakuno and Atobe) or become physically violent (as proved by Kirihara).