The last of the Martians, J'onn J'onzz came to Earth in an attempt to prevent the alien race, known only as "the Joining", from destroying it. While undercover for the majority of his stay, he eventually joined forces with Batman to destroy the Joining once and for all. It was he who, in the months following the invasion, found other superheroes and created the Justice League, and offered Batman to join. He is voiced by Dorian Harewood.

The Martian's Powers Are:
Shapechanging
Telepathy
Superstrength
Flying
Invisibility

Weakness:
Fire

Info from Wikipedia and Myself