At first, this guess may be pretty obvious -- so why even bother writing about Rachel? As you may already know, Rachel's disappearance in Life is Strange is one of the most important key plots of the game. Max and Chloe dedicate a hefty amount of their time trying to find Rachel, only to discover her body eventually.
However, in Life is Strange: Before the Storm, we might find out more about her as a person, who she was, how she and Chloe met, as well as what kind of relationship they had. Ever since Max left Arcadia Bay to move to Seattle, Rachel became Chloe's best friend. Rachel was also the one who brought Chloe "back to life" ever since Max stopped keeping in touch and Chloe's father died.
Also, ever since she disappeared, Chloe spent a lot of resources and time to distribute missing person reports around Arcadia Bay, hoping that someone had information on Rachel -- after all, both girls planned to escape Arcadia Bay and live their lives together in California.
So, it's fair to say that such a character who was so important to Chloe will definitely have an important role in Life is Strange: Before the Storm. We only heard of her from other people in Life is Strange, so it would certainly be very interesting to finally see her in person in the prequel, as well as get to know her a lot better.