Marbles is a fictional, anthropomorphic mix breed dog and Rex's little sister. She is often seen with her best friend Little Grey, their favourite pastime being having picnics. While Marbles may appear to be nothing but a young, friendly girl, she has been often depicted as being vengeful, mainly towards her older brother Rex after he tries scaring her or pulling a prank on her. One of the most notable examples of this in one of the Easter-themed stories from the original Rex canon, where Rex plants scary traps in different areas where Marbles looks for eggs. At the end, Marbles ends up scaring Rex by putting a creepy face on his door.