Marlin is a prince of the merfolk who first appears in Heart of Avalon, when he argues with sea dragon riders and comes off as an arrogant snob. While he and Emily are being whisked around Aldenmor, they become friends and he loses his bratty attitude. At the end of the book, he makes peace with the sea dragon riders after he bonds with a sea dragon named Niva and a dozen baby sea dragons (because they imprinted on him right after they hatched), and becomes a rider himself.
Marlin was inspired by Gilly from Princess Gwenevere and the Jewel Riders.
Emily Fletcher and him meet in Heart of Avalon. He does not make a very good first impression, but as they spend more time together, he loses his attitude and they develop a mutual crush on each other.