Muller is a mad scientist on Mars who is obsessed with a species called the endo-parasitoid extraterrestrial species, the Infectoid Geigermiter. He is also afraid of forks and enjoys passing out lollipops.
The Infectoid Geigermiter progenerates by seeking out a warm, fleshy mammalian, any mammalian sentient, and gives your face a "bad hug", then unleashes its larvapositor into your throat and lays its larva in your belly. Once the larva is inside, it grows up quick, multiplies and when your body can't hold it any longer, they come tearing out of your body like 1000 razor toothed bees and destroy your body in the wake. They break all your bones and rend all your flesh, as it is expunged into oxygen, leaving you little more than a puddle of viscera and tattered skin-stuffs. Then they take over the planet. They also wipe the memory of being larvaposited from the carrier.
After Jib Janeen, the Jupiter Spy infiltrated Mars by pretending to be Sparks Nevada, the real Sparks sent Felton to find a scientist to run DNA test in hopes of detecting who the Jupiter Spy is. Felton found Muller, not realizing he was a mad scientist instead of a regular scientist. Muller warns Sparks, Jib Janeen (now posing as Croach the Tracker), and Felton about the Geigermiter, and tests all of their DNA. He is later forced to admit by Sparks that he has no proof the Geigermiter exists, and Sparks realizes Muller is a mad scientist. However, Muller also forces The Red Plains Rider to admit that she is carrying a parasite, or in this case, a baby.
In Natal Attraction, Muller is found hanging around outside Felton's farm by Sparks and Croach (real Croach this time.) Later, he enters Felton's house with a Infectoid Geigermiter which he created and grew inside one of Felton's hypercattle to "prove" that they're real. He sics the Infectoid Geigermiter on Sparks, Felton, Croach and Jib Janeen, who had been posing as The Widow Johnson. The Infectoid Geigermiter is soon defeated by Pemily Stallwark, who had just been beamed in by the USSA to work as Sparks' deputy.