Rachel Prager is pretty much a mystery to us in Book 2, Inside the Darkness.

We hear Rachel’s voice from behind a glass wall. Her job in the research center is clerical and data entry. She comes across as highly efficient. She works with Jantzen Hefferly in her office in B2-Facilities Management, although with Jantzen being the brains behind the human-hybrid project, by definition Jantzen works all over the massive complex. 

Rachel makes a return visit in Book 8, Sunchaser's Gambit, although she is referenced several times in the intervening books.

We are introduced to Rachel in Book 2 of The Human-Hybrid Project, Inside the Darkness.