Mister X (J'onn J'onzz) is the Amalgam of Marvel Comics' Professor X and DC Comics Martian Manhunter. He is a Skrull metamutant with telepathy in addition to his Skrull super-strength and shapeshifting ability. Mr. X agreed to assist the JLX with their search for Atlantis in order to protect the mutants from the world that despises them. Mr. X used his psychic ability to direct the JLX in battle, and had particularly struck up a friendship with the Aqua Mariner.
In a fight with Will Magnus in Atlantis (as Magnus had framed Aqua-Mariner for an attack on two oil tankers), it was learned that Mr. X was actually a Skrull who left Mars to try and find shelter from his own race. The JLX at first saw this as deception, but Aqua Mariner pointed out that those who judge are judged themselves.