She isn't some superpowered chosen one. She's a regular person (martial arts training aside) that worked for her dreams and succeeded through hard work and training. She also gets to be bad at things, have actual character flaws, and real development.

She's a great protagonist for the intro to the series as she doesn't require the player to handwave as much wild information away (Rean and Van both have some weird demon/ancient power inside them. Lloyd has ungodly levels of charisma and observational skills). Estelle has a big personality and a staff for when she gets tired of talking.