Big, green, bald DC Comics superhero. Born on Mars, oddly enough. Has a wide variety of superpowers, including flight, super-strength, super-speed, telepathy, shapeshifting, and "Martian Vision" (heat vision with the serial numbers filed off). His real name is J'onn J'onzz (pronounced "John Jones"--how convenient), and he spends his free time shapeshifting into various human disguises, giving him whole bunches of secret identities.

The Martian Manhunter is one of those grim, brooding guys, though he's sometimes shown a really wry sense of humor. His classic costume consists of a blue cape, blue undershorts, and red cross-your-heart suspenders -- he would not look out-of-place stripping at a bachelorette party. His more recent costumes tend to be more modest. 

He is a founding member of the Justice League of America, but since there are already so many superheroes in the United States, he acts as the semi-official superhero of the Southern Hemisphere.