Ponytail is a stick figure character in xkcd. She is distinguished from other characters by her white ponytail hair.
Her name may be Joanna, as suggested in 322: Pix Plz.
A girl named Sarah with a ponytail appears in 84: National Language. However, this comic seems to be about the real-life friend of Randall.
Ponytail is mainly a filler character, showing up when two females are needed or when a large group of people are present. In most comics where she has a specific role, she tends to have more educated roles such as in 451: Impostor as an engineer.