They say a hero is only as good as his villains, which is one of the reasons I think Captain Marvel is one of the greatest superheroes ever—he's got the best villains! In addition to his archenemy Doctor Thaddeus Bodog Sivana, he's got Mister Mind, an evil alien worm determined to conquer and rule the world.
After DC bought Fawcett and absorbed their characters, Mr. Mind was relegated to Earth-2 and Earth-S, but was introduced into the post-Crisis DCU in a short-lived miniseries it's probably best to pretend never happened (in that, he was the worm in the bottom of a tequila bottle Sivana was drinking on) and then better and more thoroughly by Jerry Ordway in his Power of Shazam series. Ordway's Mister Mind was a Venusian evil alien worm determined to conquer and rule the world, the vanguard agent of a race of Venusian worms. Ordway's was less cartoonish and more realistic in appearance, looking much more like a caterpillar.
As you can see, Cornell and Woods' Mind looks an awful lot like Ordway's, although now he's bigger—about the size of a large stuffed animal—and while his body is rounder and fatter, his legs and mandibles are pointer and more menacing looking—sometimes. Depending on the context. Here he's dripping with gore, and meant to be a bit scary.
He still talks via a radio or "talk box," although it's a little more modern and space-age in design. While the original wore spectacles, and Ordway's had big, round eyes that visually echoed the original's glasses, this one was the segmented eyes of an insect, save with one in the middle being black, suggesting a pupil. The result is an eye that looks insect-like and cartoonish at once.
Cornell usage of Mind is as the first of many villains Luthor is juxtaposed with throughout the year-long storyline. Mind is working for someone else, although he himself doesn't know who or what he's working for. He (and we) just know it's something incredibly powerful, so powerful its made a lackey out of a villain who has organized Monster Societies, attempted to conquer the world and once almost ate the whole Multiverse.
He engages Luthor in a sort of mental combat, psychically imprisoning Luthor in a series of absurd fantasy sequences, including ones in which Luthor is a Promethean caveman stealing fire from super-gods, another in which he is a playing the role of Doctor Frankenstein from the James Whale movies, another in which he's a Godzilla-like giant monster and Luthor is a Superman-like superhero and, most adorably, a wild west sequence in which Mind wears an adorable cowboy hat and wields a shooting iron:
Unfortunately, it also means he's not much for hand-to-hand combat. This sequence shows what he did to the poor sap whose skull he was occupying (note the prone, headless body in the first panel), and how easily Luthor defeats him once they've escaped the mental plane for the real world:
He returns again before the story ends (in a chapter drawn by Jesus Merino), when his unseen master is finally revealed and their relationship can be explained without really spoiling anything.