Marx is a 51-year-old lion who stands 5'11" and weighs 155 lbs. In addition to the standard Felid abilities of enhanced senses and night vision, he also has the unique power to detect other Felids (though he has learned to "turn it off", since living in Sanctuary means he is constantly surrounded by over three hundred of his kind). This sense becomes stronger with closer proximity and stretches to a diameter of approximately one hundred miles. Marx's skills include hand-to-hand combat, Martial Arts E, firearms, parenting, some psychology, car and motorcycle driving, cooking, and piano playing. He has also extensively studied climate change and is a passionate advocate of recycling and green energy. Marx oversees the ecological administration of Sanctuary, making sure that the refuge creates as small a carbon footprint as possible. He feels a connection to the Earth and wants to do everything within his power to help keep it clean and safe.

Born Marcus Samuelson in California, Marx believed himself a mutant until he met Tigreer. He nurtured her dream of gathering other Felids together. Marx believes in peace and has only reluctantly learned fighting skills as necessary for his survival and the protection of his family. He is in a threebond with Tigreer and Daman; with Tigreer, he has fathered Tyger Moon-Eyes and Cantau, and he helps raise Daman's son by Tigreer, Shade.