Tonight's episode of Supergirl has introduced the world at large (you know -- besides us comics fans) to J'Onn J'Onzz, the Martian Manhunter.
So...who is he?
After years on a Martian colony, where he was a police officer defending laws on a civil war-ridden world, J'Onn J'Onzz watched his family -- and his culture -- die before being spirited away inexplicably to Earth.
Dr. Saul Erdel, a scientist and tinkerer, had developed a transmitter machine based upon ancient Martian technology -- and it had pulled J'Onn to Earth in time to prevent him dying with his world.
The process blasted him through time as well as space, and he arrived centuries after his world had died. It was later revealed that Mars was dead when J'onn was taken, killed by a mental plague deliberately started by his brother Ma'alefa'ak.
J'Onn was a member of the Justice Experience in the pre-Flashpoint DC Universe as well as a founding member of the Justice League and its longest-running member, serving in every version of the team until the late 2000s.
One of the greatest threats he faced was an invasion of white Martians, a warlike faction of J'Onn's own people who had infiltrated Earth society for years before they struck.
In the comics, he is actually an ally of Prince Jemm of Saturn, a hero who (like everyone from his world) has Martian lineage. They did, however, briefly share a common romantic interest, which was awkward for both of them.
Of course, all of that history went out the window when the Flashpoint reboot happened in 2011. In the new DC Universe, J'Onn has always worked not with the League (he was replaced by Cyborg) but with StormWatch, an organization that secretly monitors Earth for threats and has worked behind the scenes for centuries. In some ways, his role on Supergirl is more reminiscent of this, since officially the DEO doesn't exist.
Recently, we started to get hints of the new Martian mythology, since Martian Manhunter got his own ongoing series this summer.