Initially, Saki is shown to be a confident but soft-spoken individual and rarely expressed much emotion other than what was needed to get her point across. After finding the courage to defy Izaya for the first time, Saki begins to show more of who she actually is when not acting as the puppet of someone else. She is very straightforward with how she speaks and rarely minces words, even coming off as blunt at times. 

Source: https://durarara.fandom.com/wiki/Saki_Mikajima 

She won't have many lines to begin with, but as the series goes on she'll end up getting quite a few.