Maya is part of the police force's K-9 division, but she's the human part, not the dog part. If we were to list some key traits though, you might think she's a member of the canine persuasion: loyal, hardworking, scrappy, never gives up, will do anything for her partner, follows directions well, loves to be scratched behind the ears.
Okay, we're not sure about the last one, but the rest describe Maya to a T. For most of the book, we see a dog doing everything it can to please humans. Here, we see a human working hard for a dog.
Maya has to take over as Ellie's human partner after her first partner, Jakob, is shot in the line of duty. Maya tries hard to get in shape for the position and to be the best officer she can be. It's not just for herself, either; as she says, "I'd hate to let Ellie down" (20.83).
And that becomes a running theme: Maya puts Ellie's interests first. Jakob lectures Maya on it, saying, "Ellie is the best dog anyone has ever seen. You dump her like that, you could ruin her" (22.29). This is Jakob's equivalent of batting Maya on the nose with a rolled up newspaper so that she won't think of quitting again.
Maya shows us the good that can happen when we put someone's—or some dog's—needs before our own. By focusing on Ellie, Maya eventually ends up saving lives, getting married, and having a family in what little spare time she has left. Ellie dies a natural doggy death after a long, productive, satisfying life, but although Ellie is gone, Maya's life is just beginning.