All info was provided by Kate Sanders at Lizzie McGuire Wiki
Katherine "Kate" Sanders is a recurring character on the Disney sitcom, Lizzie McGuire.
Kate's the most popular girl in Lizzie's school who is the captain of their school cheerleading squad. Originally, one of Lizzie and Miranda's best friends, Kate became popular because she got a bra after summer camp was over; as a result of her new-found popularity, Kate becomes Lizzie's enemy. Kate enjoys trying to make Lizzie's life miserable, but Lizzie always manages to outsmart Kate and come out on top. References in episodes are inconsistent about her last name (either Sanders or Saunders, but usually Sanders). While Kate was mean to Lizzie in the series, she helps her out and becomes Lizzie's friend again in The Lizzie McGuire Movie. Kate has a cousin named Amy. In the Season 2 episode Party Over Here, Kate shows a slightly self-conscious side. (It also implies that she is lonely too, because she gets a call from her mom saying that she would not be home in time for Kate's party.) Even though Kate's popular, she is not truly happy.
In the episode I've Got Rhythmic, it is shown that Kate stinks at rhythmic gymnastics and gets jealous when she sees that Lizzie is extremely good at it (but later in the episode, Lizzie reveals to her parents that she actually hates rhythmic gymnastics and that the only thing she likes about it is that she is good at it). It is rather ironic that Kate would stink at something like that, since she is on the cheerleading squad and that gymnastics in general is a huge part of cheerleading.
It is revealed in The Untitled Stan Jansen Project that Kate's biggest secret (which she revealed to Claire) was that she got held back in Kindergarten, so she is actually a year older than everyone else in their grade (including Lizzie and her friends).
In the movie, she helps Lizzie in her little plan and they become friends again.=
- She was born in the Fall of 1987.