Fat Louie is Mia's black and white cat in the series. His name was originally just Louie,but since gaining twenty five pounds,he became Fat Louie. In the books, Fat Louie is orange and white.
In the third book, Mia writes detailed instructions on how to look after Fat Louie while she spent Christmas in Genovia, indicating that she is very protective of him. Louie is, according to Mia, a fussy eater. He gets bored easily and has a collection of shiny things that he thinks Mia doesn't know about in the bathroom.
Mia thinks Fat Louie is very cute, and states in Mia Goes Fourth (Princess in Waiting) that she will never love anyone as much as she loves him. In her list of people she loves in the seventh book, Fat Louie is number one.