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 was mentioned in Keeper 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 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!