Origin

Override was a mercenary for hire with a cybernetic suit, most often using his powers in service to the highest bidder, no matter whom it was and what they wanted. Override first appeared when he and his (equally super-powered) wife Aura where hired to take down Ben Reilly, better known as the Scarlet Spider. They where however unsuccessful. Over the months, Override and Aura clashed a few more times with Spider-man. 

Mayor Story Arcs

The Gathering of the Five

However, disaster struck for Override. His wife was near fatally wounded and Gregory was looking for a way to help his wife become healthy again. He was at the end of his rope, when Norman Osborn came up with a possible solution. He wanted Greg to participate in a ritual known as the Gathering of the Five. 
The ritual needed five people who would receive five magic gifts, randomly distributed among the participants. These 'gifts' where death, immortality, knowledge, madness and power. Greg was hoping to receive the gift 'immortality' or 'knowledge' to help his wife get better. However, when the ritual was done, Override received 'the gift' of death, but not in the way the participants figured.  
A demon known as  Shadrac took possession of Greg's body, gaining an avatar in doing so. In this form, Override became a skeleton consumed with mystical fire. The fire could not actually incinerate anything but caused intense pain to any who touched it. Shadraq could however melt through steel and concrete by slightly altering the spell. He also had superhuman strength and resistance to injury.

A conflict between Override (in the form of Shadraq) and a man named Gray Dolman (a man who had gained immortality in a 14th century Gathering) ended up turning Dolman into the third component of the Shadraq form. Override returned to his normal self after that and no longer had any Shadraq's powers. How they separated exacerbate is unknown. Override tried to live a normal life after that and has not been seen since.

For more info on the demon, see: Shadrac.