When you're told to picture a Martian, you likely imagine a little green alien from the planet Mars possibly named Gazoo or Marvin. In 1995, Joseph Samachson and Joseph Certa set out to change that misconception. Thus, Martian Manhunter, a/k/a J'onn J'onzz made his grand entrance into DC in Detective Comics (Vol. 1) #225.
In the original story “The Strange Experiment of Dr. Erdel,” J'onn J'onzz crash lands onto Earth. To hide his true identity, he adopts the persona of John Jones, a detective living in Middletown, US. All the while searching for a way back to his family and home on Mars.
Eventually, J'onnz reveals his existence and becomes one of the seven original members on the Justice League of America. In an unfortunate accident, John Jones is ostensibly killed in action by Idol Head of Diablo. Forced to abandon his civilian identity, J'onzz dedicates his time and energy to defeating Idol; which he eventually does.
It helps when you have endless powers including superhuman strength, speed, durability, endurance, regenerative healing, flight, invulnerability, intangibility/phasing, invisibility, shape shifting, telekinesis, telepathy, Martian vision, nine senses, and genius-level intellect.
Late on, J'onnz appeared very sporadically throughout the DC Universe before permanently resurfacing in 1984. Choosing to reassume his identity as Jones, J'onnz is forced to contradict his "death" story and explain a 20-year disappearance. Not an easy task by any standards, but he manages.
Throughout his character history, J'onnz has been edited numerous times from countless "deaths" and resurrections, to origin story retellings. One theme that has remained consistent is his dedication to helping people. From the billions of humans on Earth, the heroes of the Justice League, and (more recently) even to Supergirl.
J'onnz currently plays an important role in Supergirl's life on the hit CBS show, Supergirl. Adopting the persona of DEO Director Hank Henshaw, J'onnz uses his immense power to protect both Kara and Alex Danvers, as well as all of National City.
Man, having an incredibly powerful (and also totally sweet) Martian on your team? Sign me up.