Catherine is an interesting character. I am always curious about characters that aren't at the forefront of the narrative and I try to figure out why, as I reject the notion that there's a thing called "a secondary character" as a reason.
I once said that Brynjar is the "backbone" of the crew, well in that case Cat is the foundation upon which Anit'za can build his plans and crew. Because of the dzenta'rii's brilliance, I think people forget that the gorilla is a brilliant tactician herself. I don't think half of his plans would work without her help. They compliment each other brilliantly and have taught each other many things. Cat has played a huge role in setting Anit'za on his current path, which single-handedly has saved countless lives.
What separates Cat from other characters are the choices she has made. She didn't have to become a soldier, constantly putting herself in danger. Unlike Alric, there was an "out" for her, Cat could've chosen to work with her family on Mars but instead something has compelled her to answer the call, to put her life on the line. In an unwelcoming galaxy, where dangers are many, that is still a choice that deserves respect and admiration.
What is curious is that I don't think that Cat has such a personal stake in this conflict, like many other characters. Don't get me wrong, I'm not saying she doesn't deeply care about her friends and family, but when most of the crew is in this fight because of something that has happened to them personally, Cat is more like Awesome in that regard. I think that she, through Anit'za, sees the final goal and realizes that it is not a dream, a mirage, but something that can be achieved and I think this is what drives her, what gives her her ferocity. Her goal can be summed up in a motto that you might be familiar with - "De oppresso liber". While others are concerned with the "now", everything that Cat does is in the name of this - a future with true, tangible freedom, where people do not have to live with the constant worry that their planet might be invaded at any moment. That they won't have to see their friends slaughtered, their children kidnapped and enslaved. Maybe that is one of the reasons she has a cybernetic arm. In a universe where she could've had her arm regrown many times over, she has chosen not to, a constant reminder that her mission is worth the sacrifices that she has made. That with every battle, every scar, she is getting closer and closer. That it doesn't matter if she lives to see it, so long as that future, that dream of every soldier is realized.