Marty is first mentioned in Book 1: Keeper of the Lost Cities. Marty is Sophie's cat, and it is shown that Sophie cares deeply about her beloved pet. It is mentioned in Book 1 that Marty was Sophie’s only friend. In addition, when Sophie is about to leave with Fitz to her "own" world, she hugs Marty and assures her cat that her little sister Amy will take care of him. She didn't want to leave him behind, but she knew she had to. It can be noted that Marty has grey fluffy fur. In the fourth book, Mr. Forkle mentions that he knows Sophie found Marty in a bush and begged her parents to let her keep him. Sophie reminisces about Marty's crackly purr when listening to Iggy. In Book 5: Lodestar, Sophie mentions that Sophie once helped her human Mom give him medicine, but the amount of yowling he did made it sound like they were trying feed him poison. In Nightfall, when Sophie, Keefe, and Fitz rescue Amy, she (Amy) begs to bring Marty and Watson with them. Alden later shows Sophie his shredded sleeves from taking Marty and Watson down the whirlpool to Atlantis.
Marty is named after Shannon Messenger's cat Marty (who sadly passed away on 1/21/2019). She cares/cared deeply about him, enough to put him into the story!