J'Onn J'Onzz is the last survivor of an ancient Martian race. He is a telepath who can use his uncanny shape-shifting abilities to adapt and blend into any situation.
By altering his physical density, he can also become immaterial and pass through solid objects. Because he comes from a cold barren planet, exposure to intense heat can weaken him.
J'Onzz came to Earth to warn us of an invasion by the evil race that wiped out his own people on Mars. Although he was met with suspicion and hostility, J'Onzz refused to give up, and helped to gather together a group of heroes who could repel the invaders. This group would become the Justice League .
After the invaders were defeated, J'Onzz remained on Earth because as the last of his kind, he no longer had a home. The other members of the Justice League try to make him feel welcome, but he always remains aloof, detached and inscrutable.
As an outside observer, J'Onzz is fascinated by contradictions of the human race. When he secretly walks among us, he is overwhelmed by the conflict between intellect and emotion he senses within every one of us. Often, it is too much for his alien mind to absorb, so he retreats to the orbiting Justice League Watchtower, where he spends most of his time.
Although J'Onn J'Onzz is the heart of the Justice League, no one in the universe is more alone than he.