Information - Role
Fee becomes more and more of an important character as the storyline goes along. While she is presented with a simple backstory at first, once her heritage is revealed it makes the journey and liberation of Riesevelt a lot more personal.
Unlike Gray who's just along for the ride because he finds it interesting trying to save the world, Fee initially joins because of Mother Eizlen's hidden motive of her finding out about her heritage. This journey, as much as it is for Felt, is a journey of self-discovery for her.
Noin already knows who she is; she's the daughter of Galahad, and despises her father for serving the Empire, as well as allowing her mother to die. She serves Max because Max was like a foster parent to her, and is dedicated to him and his cause because of that.
Gray, as mentioned before, is only along for the ride because he's never tried saving the world before, and would rather have a goal to work towards than wander around aimlessly.
Poe... is Poe. He initially joined because he wants to find out something horrible about Felt so he can convince Viese to marry him. His purpose doesn't seem to change very much as the story progresses, except to escape his wife.
Felt originally wanted to save Eden, but was swept into a political war. Fee was swept into this as well, and it became a whole lot more personal as she gathered the details. Because of her heritage, you fidn out a lot more about the political background of the country.
All in all, she's rather important.