Dr. J was a former engineer for The Nova Corporation turned Mech-Gladiator after being fired for unknown reasons. He is seen only in flashbacks due to his death before the events of M^3. He created quite a few of the technologies seen and is implied to be the genius behind the non-mech merchandise.
Dr. J's story is seen through multiple flashbacks that are out of order. Due to this, the following is an assumed history of Dr. J. His story begins when he shows off sky city technology to Mr. Mech and the other high members of Nova. Impressed by this, the company makes him a prime member of the engineering team and puts him to work on cheaper transportation. The next time he introduces a simpler, more cost-effective version of the modern ship. He is once more praised by the people running Nova and given a higher position. The next flashback scene shows him discovering The Lunar Corporation. This is presumably after the creation of the cheap ships. It's assumed that he noticed the odd similarities and brought them back to Nova's attention. The final flashback is of him being fired and thrown into the Mech-Gladiator circuit. Here he invents Rivet to help him cheat. He is quickly discovered by Nova and executed.