There is some confusion about Kate's surname as it has been given as Sanders or Saunders in different episodes. In the novelizations of the shows and movie her name is written as "Sanders".
Kate was formerly best friends with Lizzie, Miranda, and Gordo since Kindergarten. The summer before they went to Middle School, Kate went to a camp of some sort, and when she came back, she got a bra. From then on, Kate became Lizzie, Gordo and Miranda's enemy. Kate is mean, obnoxious and egotistical towards Lizzie and her friends (as well as other students who aren't popular or good-looking). Kate uses her popular "power" as a form of bullying and disrespecting as well as showing off to everyone. It is shown that Kate is a cheerleader and this is her main source of "power" (being the cheer captain). If she is kicked off the team for any reason (which happened temporarily in one episode due to an injury), she becomes and remains "unpopular" until she gets back on the team. She often acts sweet and innocent towards teachers, students' parents and other members of authority and then goes back to being her mean self as soon as they leave. Kate can sometimes temporarily be a better person with the teachings of Lizzie, as seen in Kate's Big Adventure, where Kate and Lizzie are working on a project together. Kate's secret is that she was held back a year in kindergarten, and is one year older than her class. It is shown on I've Got Rhythmic she is terrible at rhythmic gymnastics, a very major sport in cheerleading. In The Lizzie McGuire Movie she seems to become a better person, learning the truth about Lizzie and helping her instead of ratting her out (like she normally does). She proves to be a loyal friend when she wants to, covering for Lizzie.
Amy - (played by Hilary Duff's sister Haylie) is Kate's 18-year old cousin. She is as every bit as mean as Kate and acts very irresponsibly as shown in the episode "Party Over Here". She appears in about 4 episodes. Amy is the only member of Kate's family who is seen.
Mom - Kate's mom is mentioned only on Party Over Here while she is talking to Kate saying she can't come to the party.
In The Lizzie McGuire Movie, the trip chaperone tells Kate to separate from Ethan Craft after an argument about spaghetti and how Kate won't eat it. Kate responds to the chaperone with: "Oh, we did, Ms. Ungermeyer, and thank God." Towards the end of the movie, Kate and Ethan have a romantic moment where they eat spaghetti together.