Maria Antonia Fantasia Philomel (nicknamed "Marietta") is the self-insertion character Mary puts into her version of The Heterodyne Boys and the Turbines of Atlantis. Marietta (as Bill Heterodyne calls her in the revised story) is a bit of a Mary Sue, in the same way that the Castle Wulfenbach is a bit large.

Like most Sues, she's handy to have around until your diabetic coma either takes you out, or you want to throttle her - she's easy on the eyes, can pull the perfect tool for the job out of thin air and is very hard to kill.

She is the princess and sole survivor of a mysterious yet famously powerful Spark tribe, seems to have delicately pointed elfin ears, has eyes which change color from violet to silver to who-knows-what; and comes armed with a spanner, an ice ray, a electro-harpoon and a magical amulet — although where she stashes them in her snug and skimpy outfits is anybody's guess.

Maria Antonia Fantasia Philomel has a mysterious and portentous tie to a girl named Raven Moonsdaughter.