Rema Baxter is the snobby girl who is the original owner of Lulu's pony, Snow White.
After Lulu rescued Snow White from a fence in the Baxters' yard, Rema and Lulu made a deal that Lulu would take care of Snow White while Rema is away at boarding school, since Rema's parents don't have time to take care of her. When Rema came back from boarding school, she would have Snow White.
However, when she comes back in the summer, she no longer likes riding or caring for Snow White. She thinks Lulu has damaged her horse. Lulu and her friends think Rema is now too tall for Snow White, so she should have a larger horse, not a pony. Eventually Lulu and Rema come to an agreement that Rema will sell Snow White to Lulu, so Rema will have enough money to go to summer camp with her boarding school friends.